maybe i'm misunderstanding the situation, but if people aren't following through on the tasks they commit to, couldn't they just be approached to see why, and to see if something could be arranged that works better for them? you could also let them know in as non-guilt-tripping a way as possible that the collective is really counting on them, and that if they're too busy or just feel like shirking it would really benefit the shop for them to let someone else step in who's able to commit.
the only rule we have that gets you an immediate boot at our shop is physically violent behaviour. even for stuff like fucked up racist/classist/sexist shit, we try our best to work it out by communicating, so i'm not sure i see the need for booting someone just for being a flake.
macho