Heh, we just got caught up in an everlasting problem: suppression Vs education ;)
Sarin, sure you have a good point, yet I'm a bit too old to have faith in people ;) A great polish writer, Stanislaw Lem, once said: "I haven't knew there is so many idiots in the world, until I started using internet".
I wouldn't go so far, but... Most of the spamming users of wlodb are children. And if it comes to children (well, adults I'm afraid too...) you can "force" them to obey the rules in three ways:
-not leaveing a place for other behavior (suppression).
-giving a guidelines how to behave (simple education).
-creating a community that gives a good example how to act properly (education through good examples).

Sure, suppression ruins morale, but I'm afraid is the most effective way over a short period of time.

Simple education needs authority. Someone who don't have authority won't be listened. Children in the internet feel careless, they feel like outside their parent's influence, thus they act chaotically, disobeying guidelines. Internet, nor any forum mods or admins, nor any site guidelines are not authoritative enough to make children obey those rules. It will affect only small percent of new users.

Education through good example, as an additional way, works better than just simple education, but it needs a whole community to give a good example. I mean, most of the forum should have small signatures. So far it's unreal because most active users have overgrown sigs. If new user sees a huge siganture in every second post he just follows the trend, and makes one for himself.

I think the best idea so far would be just to make a profile option to show or hide all signatures AND start educating new and existing users how to use signatures, and how to keep them reasonable. Still it would need a massive, active AND CONSTANT education.
And because I'm an old grumpy unbeliever I just don't think anyone would keep up this Sisyphean task for a long period of time. Pople can shine like bright spark in the beginning, but all sooner or later end up burned out like an ash.