rsvpBOOK has 2 versions of the Admin Dashboard.  Which one do you have?

The Accept and Decline buttons are the Submit Registration and Not Attending button labels, respectively.

1. You can hide the Not Attending button by checking the Hide this Decline button on the registration page checkbox
2. You can leave the Not Attending button visible on the Edit Registration screen so that registrants can return and Cancel their registration

If you require a purchase to attend your event, you should NOT Hide the Decline button - otherwise, someone could be required to make a purchase but they are not attending; you probably don't want to create this scenario.