Given the number of times (especially since I'm on FT in the wee hours) I've had to RBP new FT members who have spammy signature links, I think this is far from a negative move.

90/90 is probably a bit excessive--spammers usually have 1-5 posts, so 30/30 would probably be a good compromise. Generally, though, it seems to take much more than 90/90 for people to realize what a signature is, so it's probably not much of an issue...