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Once they are registered, they are registered via the connector, and I don't think there is any way to reject someone after they've registered. So even if you changed the priority status in Marketo and were allowed to change their program status to rejected, their status likely won't change in WebEx.
That is why rejected comes before registered in the standard set of statuses. This way, you reject them before Marketo tells WebEx via the API that someone has registered.
This has come up a few times, so I will talk to the product team to see if they API would allow a reject after registration. It may be that the API to do this doesn't exist. Stay tuned.