How do I setup this anti-troll feature

[kau]

My forums are having a tough time with trolls, posting really vile pornography and such.

I think a way to slow it down would be setup a Promotion so after 5 days a registered users becomes a full registered users. Until then though they have a few limitations such as they cannot use IMG tags and have a longer wait before they can post.

I also want the promotion tool to delete the user if he has 0 posts at the time of promotion.

How can I do this?

I guess I can do all that through promotions but my question is how do I shut down the [img] tag option for a certain user group.