[exim] change subject with user filter

Hello

My ACL adds special header to each mail which is send from server listed in
Spamhaus RBL.

Untill today I've changed subject of such mails with system filter, but some
of my users do not wish to recive emails with [spam] in subject (it was
added by my system filter when it finds special header inside the messages),
so I would like to ask if there is any possibility to change subject of
emails based on user filters?

For example - users tick some box in their email control panel, saves
preferences, it adds special command to user filter file and - subject is
changed. If he untick box, special command from user filter is removed and
no subject change will occur even if special header is inside scanned email.

Any help would be appreciated!
Mike