Is there a way to automatically block links in newbie sigs?

[littlefuzzy1]

If something like this was implemented, it might cut down on a lot of spam posts.

Say, either a newbie with less than 5 (or 10) posts can't have a signature at all, can't use the "url" tag, or can't use html code or forum code in his sig at all (blocking images AND links)
Or, do the same thing, but limited by time instead of posts - 2 weeks to a month or so?

It might even work to have a mininum number of posts AND a 1-2 week time frame, as I've seen several spambots that have made 4 or more posts.