I think a simple way is just make it the same as the "unlock" to WD vesting steem. I think its like 10X what the system gives you on sign up.
So all they'd have to do is make 3 accounts then right? :)
replying here due to nesting. a really simple fix would be to make flags not count until some critical threshold between positive votes and negative votes is reached. So for example, say 5:1... so if an account had $10 worth of upvotes, flags would be invisible and have no effect until there were at least $2 worth of downvotes. But at $2, the full amount of flag would kick in. This way, users could still cast their negative votes, and they would still count (at least to get the post to critical mass, which is still something) but if they were in a vast minority their vote would be ignored as an extreme outlier.