I do agree that jr member signature campaigns or even low ranked members joining campaigns such as members is imposing a problem, because it's so easy to earn that merit. However, I very much doubt theymos would implement any kind of restriction, and I the projects advertising don't really care about the quality of the forum only their project.
Given that
LoyceV's data have shown that the 1 merit restriction is working, I don't see why we wouldn't increase that limit to 10, at least on a trial basis. It would improve the signature spam situation further, whilst also making it much more difficult to buy merit or have users ranking up their alts.
The other option, as you say, is to have some restriction on the projects themselves. If they employ spammers, then after a certain number of offences they would be shutdown or banned. This would be much more labour intensive though, and it would likely require and entire team of people to monitor every signature campaign.