It's not just the signatures - they put links all through their posts. The annoying thing is it's getting worse by the day. Six months ago we were just clearing 10 spammers a day (users require moderation approval for their first two posts on our Forum to keep the spam at bay for normal users) and now it is well over 200 per day. They between 5 moderators, we barely participate in the forum anymore - we just clean it.This forum could probably ban links in signatures and links in the first few posts.
Oh, and we mostly see these links as: advertising knock-offs from China, selling dodgy software from Russia or sending you to a site laden with Trojans to infect your machine. I don't think there are many legit businesses who are using such methods to gain business through such methods.