Hackers and trolls with access to Zoom meetings links are "Zoombombing" -- infiltrating Zoom meetings to screen share graphic, inappropriate content.
Zoom has updated default screen sharing settings so only hosts can screen share, and participants can request screen sharing access from hosts. See instructions to change your default Zoom screen sharing settings.
You can also prevent unwanted visitors to your Zoom meetings by not sharing Zoom links publicly. See other suggestions to keep trolls out of your Zoom sessions.