Here's another solution. Although it's dependent on the two instances of (BF:XX) being part of the topic and no other occurances of the ( and ) characters. I don't know if that's a potential problem for you, but given the format you posted, it's another solution none the less.
$remove($gettok($gettok(%t,2,41),1,40),$chr(32),:)
Doesn't matter what XX is or if a : or space is present (both are removed as you can see in the code). As long as the topic stays in the same or similar format, it should always work.
