Once the single payment has been made, cancel it - it then becomes "inactive" (it doesn't affect the payment already made providing it's gone through). If a future payment to that seller is required just set up a new payment as you usually would. The recurring facility is a Paypal feature to save users the "chore" of setting up each payment for suppliers you use frequently. If you don't trust Paypal to operate it (which I don't) then do as above.
P.S. Your post reminded me I still had one active so off I went and cancelled it. It certainly works for me.