Sadly this just often leads to more abuse as we have already seen with a group of people on one of your campaigns just trading merit between themselves to both get on to your campaign and get the bonuses. Whatever systems we put in place people will just try find ways round it if they can and those with multiple accounts can often do so quite easily (and the ones that don't are often just engaging in merit for merit trades). I still think the only way we're going to combat abuse is to just remove signatures altogether and make people pay a hefty sum to get one. Really can't see many people shelling out $500+ on more than their one or two accounts.
It isn't really the allowance of signatures that is the problem, but the use of them. Many members don't care about their reputations, so they will promote any old thing, as long as they get paid to do it. Signatures should be a reward and a thank you from the forum, and not a right for any member to scratch a few coins by posting and promoting rubbish.