I'm going to interpret this question as: "In a hypothetical anarchist society, what would prevent warlords from springing up like in Somalia?"
It wasn't as though warlords in Somalia "sprung up" out of a vacuum - there were institutionalized concentrations of power before the government collapsed, and those concentrations of power replaced the state as the dominant force in that region. Warlords are just what happens when you dismantle the state but leave capitalism (and other forms of institutionalized power imbalances) intact.
Anarchism isn't just anti-statism, it's an opposition to all forms of institutionalized hierarchy and authority.
EDIT: For more perspectives related to Somalia, see responses to similar questions.
http://anarchy101.org/284/how-respond-idiots-that-think-that-somalian-society-anarchy
http://anarchy101.org/2722/is-somalia-an-example-of-an-anarchy-and-if-not-why
