No, you will not get put on "a list". You are free to return your vehicle early if you want. The rental company will charge you as per their rules, but more often than not, on a pay-later rate you will end up getting some sort of credit for the unused days.

Note that this is a simplistic explanation and other factors can come into play, so please put away your flamethrowers folks.

Also, keep in mind that you are not likely to get any credit on a prepaid reservation.