People can already turn off signatures but he's making an argument that they lower the quality of discussion, which you can't really turn off unless you start blocking people.
Also, you can turn off ads and avatars which is nice. Signatures provide the site with a lot of adspace banner views which I think theymos wants to keep (as would be expected), even if there is spam.
The advertisement theymos sells are not placed in signatures. Are they? If signature campaigns were removed those ads he auctions off every month would be more valuable IMO.