I think we should consider eliminating signatures all together. Or at least ban advertising in signatures.
THe problem isnt so much the signatures themselves, they can be blocked, its people creating a shitload of accounts and posting countless meaningless 1 or 2 line messages to bump up their activity level only to sell their signature space. It really lowers signal/noise ratio.
Start reporting the meaningless posts, if they're really just for post count then they'll be deleted.