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.