Hi Kathy
That's one of the limitation of the extension and I haven't had a chance to address it yet. Right now, in Events Booking, we don't have a way to change / remove a group member due to the complicated of fee calculation support in the extension.
I will try to address it, however, to be honest, it is quite difficult and I am afraid of I can only have it improved in 4 to 6 weeks
I haven't tried but maybe you can try this solution, option :
1. Go to Events Booking -> Configuration, find the config option Include Group Members record in Registrants Management, set it to Yes
2. Then edit that group member record and try to change the status to Cancelled
Hope that works. I am too busy at the moment and could not try. Please try it and update me with the result
Tuan