Why do you want to purge records?
You can certainly go through the Member tab and remove people from the Program or even delete them, but I would avoid automating this. It sounds like you could easily delete a client record.
form prefill has only limited utility to prevent dupes - marketo will dedupe on email address regardless of prefill.
It would be better to use the Possible Dupe filter+Member of Program, or setup a deduping process for the entire instance.