This is not a good idea

1. Do you realize how many Bitcointalk users are earning money for their posts while they are unemployed? I know that many, many of them.

2. There always will be a spam somewhere - you cannot get rid of it just by disallow ads in signatures - sure, you will get out few spammers, but there will be more and more even with no adv in sigs.

3. If you will disallow ads in sigs users like me will be very sad and mad at you because this is the way to spend very nice time @bitcointalk and earn money at a same time - I spend @bitcointalk about 3-5 hours daily - sometimes more.
 And sorry, but if I will lost my job as a sig seller I will not be able to spend here so many hours ... because I will need to find a way to earn money

You might say to me "So you are posting just because of earnings?! Ban him!"
I will say to you "No.. I am posting so much because I love it - and I need money too so this is so great to earn money while posting!

I believe people like me can make @bitcointalk better - bigger forum ever!
So Bitcointalk is earning something from those Sig sellers too! Yes? Yes!

anyway... I hope @bitcointalk will never disallow ads in signatures ...