Create a user profile field, allow them to input forum IDs separated by commas that they dont want displayed.
Then wrap a conditional around the link in navbar
Code:
<vb:if conditon="$bbuserinfo['fieldxx']">search.php?do=getnew&contenttype=vBForum_Post&exclude={vb:raw bbuserinfo.fieldxx}<vb:else />search.php?do=getnew&contenttype=vBForum_Post</vb:if>
Pretty sure that is the correct conditional use. But you get the idea.
replace the
xx with the id of the custom user profile field.