You can edit the donor's Pledge Schedule to stop future Payments from auto-processing.

1. Search for the donor’s account
2. Click the blue Journal link under their account header
3. Click on one of the Payment journal entries
4. Click the Pledge Information section on the right hand side
5. Click the Original Pledge link
6. Under the Pledge Information section on the right, click Modify Existing Schedule
7. Select the Process Type as Manual
8. Optional: If the donor will not be fulfilling the rest of the Pledge, mark the box for Write Off on the left hand side as well
9. Click Save