On a PPS program, I can see the problem. I would just jack the price of the one time membership option to be higher than your average PPS affiliate payout so you at least make a little from it. It would also drive them to a recurring signup as a lower cost alternative, if getting them into recurring is the goal.

If the difference between the recurring and one-time membership is less than $10, the ones that hate recurring will usually go for the one-time. So make them pay out the ass for the option of one-time billing.