You setup a profile field to reflect yes/no response/drop down. Then surround your shoutbox code with this:
HTML Code:
<if condition="field[X] == 0">
// Shoutbox code here
</if>
X is the field id from the new profile field you just added. I also assume you are familiar with vBulletin. If you need more direct instructions just let me know.
The == 0 means if they have it set to NO then it will show, otherwise it won't. I can't remember if it should be:
HTML Code:
$field['X'] OR $field[X]