ISPs before being sued are asked to 'act' against you, when threatened. This is usually a warning to cease and desist your activity e.g. stop (I have been in ISP biz for decades).
There are a few steps you can take, first what has the ISP said specifically? (Ask to see the email complaint against you..for clarification).
Like in a car accident always act surprised and never accept responsibility for the act only agree to stop doing it (once you know what they want).
Once you have received the complaint you can send a letter like this ...
Dear ISP,
I am in possession of the copyright complaint against me. As you can tell I have never been notified of this sort of activity before.
I am told that as my wifi connection is not password protected, this may have lead to someone else using something called an IP address ??
I will work to address that, in case that was the culprit
..... this also gives you a get out clause if busted again...
However, on researching this, I note that downloading from some sites for movies and tv shows are legal and supported by the industry (e.g. UK Nova).
How are consumers and of course ISPs supposed to know the difference? I will of course comply as always with all terms and conditions
This may work on a few fronts but there is no guarantee, I am not a lawyer but dealt in stuff like this for decades (and got busted recently myself) someone must have been using my open wifi port (damn thought I fixed that).
Again always keep the tone friendly, to the ISP they are trying to stop being sued themselves, if early enough in the procedure, you should be ok.. Once you send the email to the ISP per above, do not be surprised if they do not respond, as they have done their job by threatening you.
Remember in order of strictness, music, then movies, then software, then tv are the order of aggressiveness in chasing downloaders.
If you do it regularly consider a global VPN (changes your IP address) or just stop.
Hope that helps..