I would like to know how to remove the "FW:", too, because we have to
forward a lot of emails into a repository that gets indexed and the
FW: messes up the indexing in the repository.
I hate to say it, but, it sounds like you need a new indexer more than anything
else. Removing the 'FW:' isn't easy and won't always work, as it requires
editing the form itself, which forwarded messages won't use. Even if you
published the form globally and made everyone use it, external email wouldn't
use it and you'd have simply poor behaviour.
The best solution really is to figure out why the indexer gets messed up
by the FW.