Here is what I usaly do when I need HTML enabled on a forum, and have all people from a usergroup have access in posting.
- Create a usergroup that you plan to have access to post HTML.
- Create a forum with HTML enabled.
- Select Forum Permissions from menu Forums & Moderators.
- This will show a list of forums and custom options per usergroup per forum.
Find the forum that you made and edit the usergroups and turn off all options in "Post / Thread Permissions" for the usergroups you want to stop from posting.
Everyone you move to the custom usergroup can post in that HTML enabled forum with out problems.