Clones & registration hassles

[prof3ta]

Hi, the last few days i have been getting a lot of clones coming to my site to post innapropiate material. I have been trying to fight them off with ip banning but they use proxys, and user moderation upon registration i not an option. The only solution i can come up with is having new users post at least 50 posts before being able to use bbcode. Is there a simple way to do this?

Or do you have any other solutions to fight off clones?