There is no reason you would have to cancel it and there is no process to do it really. If you must, send a registered letter to the Canadian embassy where you live stating that you want to cancel your PR. Include your PR card. I am sure they will have no idea how to handle that though. It would be easier once you are outside Canada for more than 3 years and do not meet the residency requirements any more. Then you could apply for a travel document to the Canadian embassy and write on it that you don't actually want it, you just want them to revoke your PR. Then they would because there is a process for that.