I'll share the code. Here you go... Header template, just above
$_phpinclude_output
Add:
Code:
<if condition="$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.