Unless once someone 'accepts' an offer from one of these micro-VC firm they are entering in a binding contract that has some sort of penalty for breaking it, could you not simply allow candidates who have accepted elsewhere to still interview with you? If you like them and accept them, then they'd get to choose whether they'd like to stay with their existing micro VC or jump ship for YCombinator. Granted, filling one of your limited slots with people who have opportunities elsewhere may be a waste of your time. But that would at least mitigate the problem of people 'stealing' away your interviewees.