I don't know about a hack, but you could just restrict them from seeing it with <if> conditions (Think I should release this as a hack? :p).
1) Open the "contactus" Template.
2A) Find: <form action="sendmessage.php?do=docontactus" method="post">
2B) Add Above*: <if condition="!is_member_of($bbuserinfo, 8)">
3A) Find: </form>
3B) Add Under**: <else />
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="tcat">
<span class="smallfont" style="float:$stylevar[right]"><strong>$vbphrase[send_email_to_forum_administrator]</strong></span>
$vbphrase[contact_us]
</td>
</tr>
<tr>
<td class="panelsurround" align="center">
Hmm, GTFO.
</td>
</tr>
</table>
</if>
* Change the red number to the banned usergroup ID.
** Change the blue text to what you want the banned user to see.
Hope this helps :).