I believe the only way at this point is to share your screen yourself (organizer). Thus, others can not take over w/o your confirmation to my knowledge. It seems to be missing in the meeting customized settings (to disallow taking over screen share).
Hm but it is not professional enough. It should not be too much work to set it in the settings for activating and deactivating. Such small bugs make the great app unpopular.
Maybe it was not clear enough: The organizer always has full control on who may share. It works perfectly that way.