Economically, capitalism is nothing more than the private ownership of capital; and thus it is entirely compatible with not only voluntaryism but also any other kind of anarchism, as it takes (imposed) rulers to stop people from making such private trades or claims.
"Capitalism" is frequently used to mean "the monster under the bed that I fear", however; and "capitalist" is privately defined by such users as "a person who has something I don't and it's not fair" (thanks to Chris Lyspooner). On it, or rather it's specter, are all the ills of the world blamed.
Do not confuse capitalism with "crony capitalism" or "corporatism", which is the (incestuous) marriage of government and business. A business with one or one primary customer, that customer being the state, is no more "private" than the state military; and the same goes for one whose existence or profits are protected by state violence (i.e., those that depend on IP laws, prisoners delivered to them by laws against victimless "crimes", state-enforced monopolies, and the like). Voluntaryists are in no wise supporters of such corporations, or, at least, not their privilege (a business providing trash pickup services is fine; one given an exclusive contract to operate in a city is not, because of the state's threat of harm to other competing businesses, and not due to the nature of their service, which is a useful one in isolation.)