They can be. I've been studying Eastern Philosophy recently, and reading much of the works of Lao Tzu I think he truly holds a spot in the libertarian tradition, he would be a prime example of a libertarian who is an anarchist. He strongly opposes any form of state interference and regimentation. I'm actually surprised libertarians don't bring his name up at all.
I tend to think most Libertarians that find a role for a minimal Government would fall into the category of Minarchists; much like Mises, Rothbard, Bastiat, Rand, Nock, Hayek, etc. A minimal anarchist State, smallest government possible, where it's only powers be negative ones. Libertarians don't necessarily reject all forms of Government, those that see a proper role and hold consistent Libertarian Principles would probably ideologically find a Minarchist state most suitable.