It doesn't make much difference. It's still reprehensible to make it hard to cancel.

Let people cancel, confirm they've cancelled, and then ask for feedback. Put a feedback box on the confirmation page, imvite them to reply to the confirmation email. If you really must know, send a letter, and make one phone call (leave a message when they don't answer).

Anything else is abuse, and they're leaving you because you're abusive.