The comments stay with the course. Enrolment has to do with access to the course. Deleting a single student's activities could create real havoc. Ooops there go the pages Chris created in the Wiki.
As per PayPal, I am not really sure. But I would say, the person pays via PalPal and then is allowed to enrol for a specific number of days. Then when their enrolment does expires they are no longer enrolled in the course. PayPal has nothing to do with that. I assume they can re-enrol in the course which will involve PayPal and the process starts all over again.
Best Chris