Then you're effectively banning someone's existing account, which they could have been using for years at that point. There's a lot of potential content there. Not a great feeling for anyone.
Seems like the simplest solution is to have some basic password requirements.
I think the OP and other posts here have demonstrated that password requirements won't solve anything given a sufficiently incompetent or not caring user. If the platform provider cares more about the security of the account than the account holder there isn't much you can do.