The only real solution to freeroll whores is to only use poker rooms where YOU, the person creating the freeroll, can send in a list of all players you want to participate in it. There’s a few rooms out there that do that – I believe Paradise used to, I think Boss Network sites have that capability, Full Tilt possibly?
Also the only other method would be to have a freeroll, prizes announced as $0, and then after the event transfer the cash to the players yourself. That way if the person who won the freeroll grabbed the password from somewhere, you can just skip him and pay 1st prize to the legitimate player who finished top.