[Dennis Olson]

I'll share the code. Here you go... Header template, just above
$_phpinclude_output

Add:

<if condition="$bbuserinfo[usergroupid] == 8">
   <div align="left" style="color: red; background-color: white; border: 1px solid black; padding:2px">
	<b>Your membership has been terminated.<br /><br />

        <if condition="$bbuserinfo[fieldX]">
           Reason: $bbuserinfo[fieldX]
        </if>
        </b>
   </div>
</if>

Where "fieldX" is the assigned name of your custom private user field.

Also assuming usergroup 8 = your BANNED usergroup.