I regret getting a 36mths 0% interest free credit card instalment plan for a service that I no longer need. Is there no way to stop this without repaying in full?

2 things you can do:

1. Talk to the service provider that you wish to cancel the service and if they agree, check the lost that you will be incurring (usually full amount less amount / charges of services that you have consumed), which is a lot but better than nothing or better than continue paying something you don't need.
2. If the service provider do not agree to any form of cancellations without partial refund, write in to CASE and go down and talk to the officer with your receipts, invoice, payment etc and they would be able to provide alternative options.

Hope you may cancel the plan and get back some $.

Cheers

No, usually you will have to pay a penalty to break the installment plan.