Sorry, but I think that standard Moodle does not provides a solution for your question.
The PayPal plugin was never developed with monthly payments in mind, but with multiple payment providers and enrolment methods in mind. So every payment plugin only presents a payment option when a user is not enrolled in a course.
Have you tried to delete directly in the database the mdl_user_enrolments record for a payer? If this workaround is doing the job, it is no big deal to write a local plugin that deletes one month after payment his/her enrolment record.
Of course, test. test and test this workaround, because I do not have a clue about the possible side effects....