has the author, who appears to read hn and hopefully therefore this, considered that the reasons for the apparently automatic ban might be malware on the machine?
In any case, I'd recommend against changing your MAC address because when you do get a hold of the network admins they may be rather disgruntled that you're violating network policy (almost any university that registers MACs prohibits MAC spoofing, otherwise you could just use the MAC of somebody that you saw on a different subnet).