If you wish to cancel a Continuous Payment Authority, you should always write to the company concerned, or follow up a telephone cancellation with a letter sent by recorded post, just for your peace of mind.
In the unlikely event they don't act upon the letter, you should immediately open up a dispute with your card issuer, and explain that you withdrew consent for them to continue to charge your card.
They should credit the money back into your account immediately while they investigate with the retailer (although this may depend on the card issuer). If it is a monthly subscription, you may have to continue to dispute individual transactions until the matter is resolved, which can take time.
You're right about the lack of controls though, and it shouldn't be the cardholder who needs to do the running around.
What happens, by the way, is the retailer simply enters a charge in the normal way. There is nothing to say it is part of a CPA, so your card issuer don't really have any opportunity to review the transaction. They could probably set up some kind of system whereby you could report a problem retailer and block payments to them specifically, but I'm not aware of such a system being run by any card issuer or scheme.