One "nice" thing about comment spam, is that it is far more annoying, and users have some tools at their disposal. We will cross that bridge if need be. I am starting to like the idea of rate-limiting promotion to once per person per day.
I'm on the other side of this fence. If there is a great comment thread with several really good ideas, or several posts with excellent comments, I don't want to be told I cannot reward that person for their quality content due to some arbitrary constraint imposed by the software. Participation should not be punished. However, I could see limiting a user's number of promotions for their first month of membership, or preventing them from being the first person to promote a post... that could limit the spammers enough to discourage them. I am starting to like the idea of rate-limiting promotion to once per person per day.
This may be too much complexity, but we could allow you a daily amount of "welcomes" based on how many badges you have. If you only have one badge, you get one welcome. If you're kleinbl00, you get a thousand.