[BirdOPrey5]

In postbit/postbit_legacy (whichever one you use) find the code:

<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]" $post[mh_unm_username_history]>$post[musername]</a>

Then "wrap" it in an IF conditional that says IF your in a particular forum display the Username as Anonymous, otherwise display the username... the code for that would be:

<if condition="$thread['forumid'] == 99">
Anonymous
<else />
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]" $post[mh_unm_username_history]>$post[musername]</a>
</if>

Replace 99 with the forumid of the forum you want this to apply to.

Note- after thinking about this a little more you probably also want to wrap the same if tag around all the other fields in the postbit so that you can't tell who a user is by reputation, location, age, or any other field. You could make 1 long if statement that only shows "Anonymous" and otherwise processes as normal if not the anonymous forum.

You'd also want to disable signatures in the forum or use a similar if statement around the signature to kill it too.