From the 6.3 Changelog:
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Modified the banuser function so a user cannot login at all when they are banned. If they are currently logged in they will get a message that they are now banned on the next screen they visit.
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But it's towards the bottom of the changelog - so it probably wasn't in b1 yet.
That mod won't work (or be needed) with 6.3 as the ban check was moved from every script to part of the authenticate process.
Upgrade and you'll be set.
