Removing domain name in freeradius

Hi all, Currently when users connect to our WLAN they enter their username thus:- firstname.lastname@mydomain.ox.ac.uk Is there a way I can strip everything after the @ out (ie the domain) - so they are forced to authenticate against the domain I specify. At the moment in my test environment, as long as I DONT specify the domain it works - so I'm looking to strip out the domain name if they DO specify it. Cheers, Mark