I think this will be hard or even impossible 
i thought its like this
ADMIN
/ \
(e)Playdota.eu (f)Playdota.eu
/ \
(e)Playdota.eu-bots (f)Playdota.eu-bots
So
When we whisper playdota.eu @ eurobattle.net (/w playdota.eu !m F DoctaSpock <msg>) he asks the admin that it gives the command to playdota.eu @ fawkz to whisper docta spock ( /W DoctaSpock <msg> )
if its like that what i said than it could be possible

than playdota whould write him not you so !m E <name> SPAM SPAM SPAM constantly it could end flooding your screen and bot would also lag whit that much players
than playdota whould write him not you
i dont understand you :S
there could be a command like !mignore <user>
(so when the spammer writes /W playdota.eu F doctaspock SPAM SPAM SPAM
the bot will answer , sorry but this user ignores you
and i m sure after he get that msg he wont try to spam him more )
and why would someone spam someone xD?