If you totally stop working and receiving a T4 then you will automatically stop paying CPP.  If you continue to work and no longer wish to contribute to CPP, you will need to fill a CPT30 as seen in our TurboTax FAQ: The Canada Pension Plan

If you are at least age 65 but under age 70 and still employed (or self-employed), you must complete some paperwork to stop making CPP contributions.

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