Users can individually disable the rich text editor by unchecking "Use the rich text editor to create and edit messages" in their browsing preferences. There isn't a way to disable it globally though. Besides, doing so is like using a sledgehammer to crack a nut. If you are getting a lot of [/B] tags being added in replies, there's another issue at play here, either a user issue or, more likely, an add-on that's causing the problems (try to disable any add-ons that might have any effect on or relate to posting messages to see if the issue disappears).