For the record Bigpond, Optus and Primus are very co-operative and when the evidence accompanying the email is substantial they have at times disconnected a user without a warning, in other cases issuing a warning and then disconnecting for a further breach; the rest (and not necessarily all for the wrong reasons) prefer to keep as many customers as they can get. Being a reseller of bandwidth is a game of very low fiscal margins. There is also the issue of the culprit complaining about being cut off to the Telecommunications Industry Ombudsman (in the case of an Australian based dispute) or similar statutory authority in other countries where relevant. ISPs don't need the grief that comes with having to reconnect a lamer because of bullying from the TIO or solicitors, etc.
Alkoholik: It is not always (and probably rarely) possible for a luser who connects with a proxy to be traced. In all but the most serious cases (eg: where a crime has been committed) it's not worth the bother and time trying.