Literacy is certainly a problem on this forum. I would really like to suggest banning all illiterate members, but that could be viewed as discrimination by me.
How do you think it can be stopped? Advertisement is a major source of income to the site.
Paid signatures can be removed. Signature advertisements do not directly contribute to the site's income; I'm sure that it can live without them.
But you've gotta admit that a large chunk of the bitcointalk community is just here for the signature campaigns.