They could have a security bot which automatically bans such users and the bot's database could simply be out of date. There is a slight chance that you may have inherited the IP of what was once a proxy server.
Another thing to check is... Do you run a proxy of your own such as WinRoute, WinGate, etc? If so then you may have some ports open which need to be closed. Most networks check for insecure hosts by probing ports 80, 81, 23, 1080, 3128 and 8080. Some check more but this is more timeconsuming too. Having an insecure host running not only leaves you liable to being banned, but also allows people to 'bounce' off your machine (inheriting your IP) to avoid bans.
Proxy detectors work by seeing you trying to connect to IRC, then trying to connect to your machine on the above-mentioned ports, then if that is successful the detector tries to connect back to itself to confirm an open path (insecure host) which then resuls in the ban taking place. A bot on the other hand just has a database of IP addresses stored in a has table. If your IP matches a token in the hash table you get banned.
As Parabrat said, try emailing the administrator of the network you want to use and quote the IP you were using at the time you got banned.
