If you do a live Yahoo! draft, then you must tell everyone to draft their keepers in either the first three rounds or the last three rounds. It doesn't really make a difference which one. But, I'm fairly certain that if someone screws up, you have to either force them to drop/trade their players after the draft to make it right, or restart the league.
Or, you could hold your draft offline, be it on a forum or in an AOL chat room, and then just submit the lineups manually into Yahoo!.
The more you can increase fear of drugs and crime, welfare mothers, immigrants and aliens, the more you control all the people. -Noam Chomsky