You can set the avatar to not show up when you're creating your vBa page (in the News options). Or as you've requested, try this:
In either 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]">$vbphrase[xs_avatar]</phrase>" border="0" /></a></td></if>
And replace it with:
<if condition="$thread['forumid'] == x">
<else />
<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>
</if>
Where "x" is change to the forum ID.