How to cancel apple service repair online?

Contact Apple using www.apple.com/support/contact if you are in a country that supports it or call the appropriate phone number on this page. You can cancel your service call (I presume you are talking about an onsite service call - the repair guy coming to you) that way.

Best of luck.

This is also included in email confirmaiton.

To avoid cancellation of this service request, you must return your original IPHONE 5 to Apple within ten (10) business days. If you no longer require service, simply dispose of the package Apple sends you.