Step 1 - Go to your form 'Dashboard'

Step 2 - Click Number of Responses  

Step 3 - In the list of responses, find the registration that needs canceled and click Details

Step 4 Click Cancel Registration

A cancelled registration will still show in Number Of Responses and in reports, and can be re-registered by an administrator. If your event has a max capacity, canceling a registration will make it so the response does not count against the capacity.