For some reason this is a very common experience in relationship break ups
The person that initiates the break up attempts to keep the light on just in case, a subconscious back up plan.

To avoid the cycle relationship and create enough space for you to move on. You must be firm and Tell him to no longer contact you. No other words are required

If you soften your request in anyway with explanations and or justifications you open the door for continued communication.

If you intention is to break off contact you can’t leave openings of any kind as anything you say other than for him to stop contacting you is a opening