The ISP may reserve the right to close your account, though it is not illegal. Read your AUP, EULA, or whatever you would like to call it, it's more than likely covered somewhere. On the same token, it may not be listed in the AUP at all, though if someone were to complain about your IP showing in their firewall logs, then you are most certainly looking at some downtime whilst you search for a new ISP. Tread carefully as your ISP admin may not care for that kind of activity.