Here, I want to take on this line of criticism, because I think it is important for socialists to understand how it gets our situation completely backwards. It is capitalism that trivializes our conception of violence, narrowing the definition so as to exclude itself and draw our attention away from the very real, physical, and aggressive operation of our economy. The task of the socialist is not to reify these ideological boundaries, but to push back against them, and expose how capitalism is literal violence in every meaningful sense of the word.
With that in mind, consider the following three ways in which capitalism necessarily relies on - and denies - things that we would in any other situation understand as violence.
1. Private property is violent
We are born into a world where nature and its bounty are, by default, accessible to all. In this state of nature, I can go anywhere I like. If I am tired, I can lie down wherever I am. If I am thirsty, I can drink any fresh water that I can find. If I am hungry, I can look for a wild fruit or I can start a garden or I can kill a rabbit. The commonwealth is a gift from God, or it is the legacy of cosmic evolution; either way, it equally belongs to everyone. In philosophy, this situation is something like what the Enlightenment philosophers called "the state of nature", or what Roderick Long and Matt Bruenig refer to as "grab-what-you-can world".
Historically, people have for all kinds of familiar reasons found this arrangement impractical; most vexingly, we run into problems when two people want to use the same resource, be it land, food, water, or something else. Thats's why capitalism has come up with an elaborate set of rules dictating who may lay claim to any given resource in any given situation - rules that we call "property rights".
As Prodhoun teaches us, what these property rights really are is a threat of violence. If I say that a plot of land is my property, what I am really doing is declaring my right (either personally, or through agents of the state) to physically prevent you from using it. Crucially, even when this right is not exercised, the threat is implicit; capitalism only works when we are constantly aware of this threat and are cowed by it.
This is violence. Capitalist ideology offers all kinds of reasons why property should not be understood as a violent institution - most explicitly, through the so-called "non-aggression principle" - but going by any ordinary meaning of the term, it is certainly violent to threaten to physically coerce someone against their will. Whether this violence is justified is another matter.
2. Contracts are violent
We are born into the world with absolute freedom to bargain with each other and make deals. By default, however, we are also able to break deals. I can, for example, promise to weed your garden if you give me a bite of your apple - and then, once I've eaten the apple, I can change my mind and decide not to weed your garden after all. There are lots of reasons why we may generally consider this to be inappropriate and immoral behavior, but it is certainly not impossible behavior.
In order to prevent people from breaking deals, capitalism relies on something called a "contract". Much like "property", a "contract" is really just a threat of violence: what it says is that if you try to break our deal, I can physically compel you to comply, or I can exact some kind of alternative compensation, again using physical force if necessary. It is, again, the very real threat of violence that makes a contract work, and capitalism needs that threat.
Again, it may be the case that the violence at the heart of contract law is completely justified; the anarchy of a world where everyone can change their mind about deals may be so immoral and unworkable that we are better off maintaining order by constantly threatening each other. Still, this rationale doesn't somehow nullify the existence of violence - it simply maintains that some violent threats are good and necessary.
3. Market activism is violent
Historically, the liberal-left has noticed that capitalism's system of property and contracts often facilitates outcomes that we would prefer to avoid. The left, definitionally, understands this as a problem with the system itself, and advocates subordinating property and contract to democratic sovereignty. If, that is, the violence of contracts and property rights becomes unacceptable to society, leftists reserve the right to nullify them through democratic referendum.
Liberals, in contrast, reject democratic sovereignty, and insist that capitalism's system of violent threats must ultimately be honored. Liberals believe that we can mitigate or nullify capitalism's adverse outcomes while still playing by capitalism's rules. This is the logic of conscientious consumption, employment selectivity, boycotts, and blacklists; in all of these cases, activists are still respecting contract law and property rights, and in fact what they hope to do is leverage the violence of those institutions towards positive outcomes.
Return, for example, to the strategy of blacklisting. The goal of a blacklist is to prevent someone from entering into employment contracts, which in turn cuts off their access to resources they need to survive and maintain a reasonable standard of living. Clearly, this strategy cannot work without property rights; otherwise access to necessities would not be cut off, because one could always just take what one needs. For this reason, blacklisting requires activists to not only maintain property rights, but to leverage their violent threats against the target. If you are blacklisted, you are threatened with a dangerous choice: either comply and regain access to the labor market, or steal necessities and risk the violent enforcement of property rights.
Once again, it may very well be the case that blacklisting can be on a case-by-case basis good and necessary, just like boycotts can be good and necessary. Only absolute pacifists deny that violence can be justified under particular circumstances. Nevertheless, whenever we are engaged in market activism, we should always be clear about what it is that we are actually doing. When we deny the violence at the heart of such efforts, we are denying the violence of property rights and contract law, and we participate in capitalist ideology's effort to veil them. Socialism does not deny the necessity of violence in ordering our world, but it does demand that we acknowledge it for what it is - and to minimize it as much as possible.