6.2.X EE, 7.0.0 M3
Steps to reproduce
1. Add a site and create a page, placing MB portlet on it
2. Create an user A and make her Site Member of the site
3. Login as A and browse to the page having MB portlet. Create a thread
4. Create an user B and make her Site Owner and Site Admin of the site
5. From other browser, log in with user B and ban user A.
6. Go back to the browser where A is logged and press "Post New Thread" button
7. Create the thread. An error message related to permissions will be shown but the thread has been actually created.
8. Step 7 can be repeated ad infinitum by pressing browser back button and re-filling in the form
LPE-12853 A banned user can still create discussion threads