[I realize that this thread is a bit old, but there has been some recent discussion elsewhere about 'thank-you' posts so I thought that I'd add some thoughts to it.]
On our group we discourage 'Thank-you' posts and other one-line replies such as 'I agree' since that creates useless posts to wade through. This can be a real problem for many of out members living in a 3rd world country with slow internet plus per-minute charges while online. So we suggest that 'thank-yous' be sent privately, off-list directly to the person the poster wishes to thank.
We do, however, allow 'Thank-you' posts when they also include new or useful information to the thread in addition to the 'thank-you'.
The real difficulty we have found with not permitting 'thank-yous' is that with some folks posting a 'thank-you' is so ingrained that they become miffed when told that's not allowed and get mad and leave the group. This is despite the explanation in our posting guidelines explaining the 'thank-yous' and other 'one-line' posts are not allowed.
Maybe it would be useful to have an optional feature added which can be toggled on or off in the settings for the group whereby each new member would have to check a box verifying that they have read and do agree to abide by the posting guidelines for the group before their membership is validated.
OK – HTH