PIP: Ending ongoing claim

I have received PIP for a number of years due to long term physical and mental illness, and after much contemplation, I made the decision to end my claim this year, as I feel I can no longer manage the assessment process.
I have written and sent two letters (one in January and one in April of this year) formally asking for my PIP claim to be ended, but have not received confirmation yet that the claim has been closed, and PIP payments continue to be deposited into my account.
Both letters were sent by Special Delivery, and were received by the DWP office that deals with my claim, so they must have been recorded in some way, but no action has been taken in ending my claim.
This was not a decision taken lightly, but I have not been comfortable on this benefit for years. The repeated disruption to my mental health each time a reassessment and face-to-face assessment came up was too much of a setback.
How best to proceed?
I have problems speaking to people on the phone because of high levels of anxiety, so am very reluctant to do this. I would rather this be done by correspondence.
Should I write another letter? The two previous letters were sent to the address of my original decision letter as advised. Is there another department I should contact?
Your advice on this matter would be greatly appreciated.