This isn't true at all. I'm not sure what kind of libertarian/voluntaryist you are, but there are many voluntary ways to have IP. First of all, IP can simply be enforced through a DRO mechanism if the society recognizes it as protected property (which is just as arbitrary as real property if you think about it). Even if the society doesn't accept it, I could have it so that before I give my book to you, I have you agree to a contract that you will not copy it and sell it or give it away or distribute in any fashion, and again I can use the DRO if you violate this.

Your statement makes about as much sense as "An non-violent act (squatting on someone's homestead), needs a violent act (the enforcement of property rights) to be stopped." This is a pretty solved problem in the voluntaryist community.