Yes, you can turn off Payment Plans for any future orders by browsing to Event Details> Edit Event in the Dashboard menu of your event and unchecking the checkbox "Would you like to activate a payment plan for your event?".
Confirm your changes at the bottom of the screen and all future orders will be unable to access the Payment Plans.
However, all past orders that have been purchased using the Payment Plan will still need to complete their payments as scheduled when they made their booking.