And tell them why.
What might be a good Idea is to put them on New Member Moderation with a count of 4
and inform them they are moderated because of the profuse thanking.
Don't tell them it's only for the next 4 msgs they send but if they don't change their ways by the 4th, put them on permanent moderation.
Or Cannot Post, telling them their posting rights have been suspended.

I think you will have more issue with people Replying to other messages with out of topic content
--

Bob Bellizzi

Founder, Fuchs Friends ®t
Founder & Executive Director, The Corneal Dystrophy Foundation