[Gray Matter]

In postbit or postbit_legacy, find:

<if condition="$show['avatar']"><td class="alt2"><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="$post[avatarurl]" $post[avwidth] $post[avheight] alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td></if>

and replace it with:

<if condition="$show['avatar']"><td class="alt2"><a href="member.php?$session[sessionurl]u=$post[userid]"><img src="<if condition="$post[usergroupid]==9">images/avatars/banned.gif"<else />$post[avatarurl]" $post[avwidth] $post[avheight]</if> alt="<phrase 1="$post[username]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td></if>

Change the red number to your banned usergroup's ID and the green URL to the URL of the avatar. Hope this works for you.

(Also, remember that this will only affect the avatars within posts, not within member profiles or any other part of the board. You will have to edit those manually if you wish to change them as well.)