You can't enforce it but you can avoid it. When choosing the delivery method select freight, don't add pick up.

In the payment field choose Paypal and cash on pick up.

That way if they want to use Paypal they will have to pay whatever you want to charge for a very expensive courier delivery.

If you already have a buyer who wants to use Paypal explain to them that you will accept it if they insist but you feel you must point out that there is no buyer protection for pick up items so they will not be able to reject it if they decide it is not what the really want as they could if they were paying cash.