Two ways, depending on what you want done
Per each style you have available to your forum. From within the Control Panel > Style Editor, add the following to the EXTRAS section of the style you want it hidden from
If you want it changed or completely removed from your forum, edit it the same way that you would normally edit any other language item.
1/ GO TO Control Panel > Languages > Language Editor
2/ SELECT the Search / Edit Language Strings tab
3/ SEARCH for "Comment Guidelines" as the Search String
4/ CLICK EDIT next to "SHOUT_INTRO" and then edit or remove it as you desire.