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
Code
.shoutintro{display:none;}

or

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.