I have a thread ban on a member but he keeps posting. I check the log and it's all correct. I do a test permissions on his username and indeed he still is able to post in that thread. What should I be looking for?
The permissions analyser would only give you a view of the user's capabilities based on what actual permissions he has. As such, as banning isn't technically a change of permissions, this usually wouldn't be an accurate reflection.
If he is able to bypass the reply ban, and you're certain he should be banned, I do wonder whether there's an add-on at play here. If an add-on has made a bad job of extending the method that checks whether a user can reply or not then it's possible the reply ban check is being skipped.
First, see if you can reproduce the issue on a test account. If you can, then it will be easier to test. The test would unfortunately just involve disabling all add-ons and trying again on the test account.