I am using 7.3.1 and set up a group for new users. In the group permissions I set the Minimum time between posts to 900 seconds. It stops them from starting multiple new threads but not from making replies every few minutes in multiple threads.

Perhaps when I upgraded the code had not been changed yet?

Hmm, I'll need to check a couple things and make sure this is working properly in our latest code and that there isn't a bug lingering around.

Ok, this definitely works in 7.4. Will test on a 7.3.1 forum, may have been a bug in that version that never got reported.