1) Find the pharmacy phone number on the pill bottles
If you can't find the bottles, you can call the deceased's doctor and ask if there are prescriptions that refill automatically and which pharmacy to call.
2) Call the pharmacy:
Ask them to cancel automatically refilling prescriptions
You can also ask the pharmacist where to drop off / dispose of unused prescription medications. This is important to avoid accidental or purposeful misuse of the medication and contamination of the environment.