With the speed those guys hit, I imagine it'd be pretty tough for a referee to stop them before they've gotten an extra couple strikes in. It's an inherently dangerous activity, and sort of hard to mitigate that fact. I guess they could disqualify a fighter who hit their opponent in the head more than n times in a given match, or even make the head out of bounds altogether. Something tells me that's not an action the UFC would take willingly, or that UFC fans would accept happily.