If signatures are ever removed, it would clear up 100% of the spam which goes on here, but would also reduce a number of active users contributing positively, or reduce how much time they spend on the forum.
I wouldn't be so sure that removing signature campaigns would cause a total removal of spam. Keep in mind that's not the only way that someone can promote their service on the forum as there are also so called "bumping services". Some of those services (at least better ones) are not only writing exclusively in one thread, but they go around and do sort of guerrila marketing that could become quite popular if signature campaigns were removed.