When viewing an event invitee from the Invitations or Registrants tab of an event, users have the ability to mark the invitee as "Declined," when they have indicated that they will not be attending an event. The option then changes to allow users to "Mark as not declined," if the invitee changes their mind and decides to attend.
According to the Events user guide (page 72):
If you mark an invited registrant as declined and you later need to change the status back to No reply, select Mark as not declined. In the grid, the Status column displays "No reply" for the selected registrant and the Register button is activated, allowing you to register the constituent for the event.
This indicates that the "Register" button should be inactivated when an invitee is marked as Declined, which is not happening right now - currently, an invitee can be marked as "Declined" and "Registered" at the same time.