Actually this is happened randomly. And few of our clients complaint about this issue. So the steps may not be identified.
Did you find such issue before to any other users? and so if there any specific reason for causing it. This can help me to debug it.
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).