Members of programs can only move forward through the program statuses. That is why you weren't able to move them from Registered to Rejected, since the Registered Program Status comes after Rejected.
In order to have the ability to reject individuals, you would set their status to 'Pending Approval' once they fill out the registration form, and then you would scrub the list and manually change those that you don't want to attend to 'Rejected', and change the others to 'Registered'.
This way, members won't get the standard registration confirmation email until you approve them.
The other thing you could do is to create a Smart Campaign to automatically approve or not approve members from the 'Pending Approval' status based on a defined set of conditions.
Once they are registered, I believe you'd need to go into WebEx to remove them.
Thanks,
Mike