I believe you can do this by turning off the advanced WYIWYG editor.
ACP -> Settings -> Options -> Message Posting Interface Options
Change
Full Editor in the "Enable Clickable Message Formatting Controls" section to
Enable Standard Controls
Note: This will limit your users to using only the basic editor. If you want them to be able to use the full WYSIWYG editor then you will have to make some template edits. I do not have the time to find those templates and suggest which edits to make, but don't worry someone else will come along and help you with that