Prevent panelists from ending practice sessions

Hello, 

We hosted a webinar and our panelist ended the practice session and some of the hosts and co-host did not realize, so the attendants were joining what it was meant to be the webinar practice. Is there a way to stop panelist starting the meeting from the practice session?

We also had some trouble letting her join her screen, we were told she had to be host or co-host to do so. Can we make sure all panelists have access to screen sharing by default?

Thanks for your help!!