I have never used Swisscom's PWLAN. Is this really an anonymous service?
yes & no: you can either use your mobile number to access the wireless lan (and it gets charged to your phone bill) or you buy the prepaid cards.
This is not very creative, but why not just check if an IP address is blacklisted before assigning it to a client? - If this yields too many "unusable" addresses, the whole PWLAN service as it is provided today probably needs to be reconsidered.
i think the problem was not a spammer, but virus-affected users. we got on blacklist several times where ppl announced the virus-propagated spam (or the virus itself) to the blacklists...
-steven