It it possible to cancel an active screen sharing as host of a room, as it is possible for mute all microphones and videos?
Yes, you can video mute the participant.
Am I understand right, that if I mute the video of a participant, his screen sharing will also disappear? There is no way to only cancel screen sharing of the participant?
Your usercase seems to be that someone does not end screen sharing, even if you ask him to. So you mute his video. If this persons wants to continue to be seen, he/she has to activate the video again. To me that sounds like a fair sort of educational punishment.