1. Contact your bank or credit card issuer to cancel this instruction.
2. You contact Shopperdisc.co.uk and say you want to cancel, and ask how they have registered you. The answer will almost certainly be an online purchase from someone like Interflora, or Amrose Wilson, who have offered a discount off your next purchase if you sign up.
3. Contact the company from which you made your purcahse and complain to them, and tell them that you are no longer going to buy from them.
4 You consider whether to make a claim via the Small Claims Court for refund of the amount, though as this will cost £20 you may not consider it worthwhile.