Here's how to delete/remove a guest:
Open the Main Gallery you shared with guests - skip this if you're already in the main gallery. Example of a main gallery highlighted below:
Click the Menu button at the top right (purple circle with 3 white lines) to open the "Gallery Menu"
Click "Guest List" on the menu list.
Click on the name of the guest you want to remove - use the search box to filter if you have a large guest list.
Click the trash icon at the top right, then confirm delete. If the guest has uploads in the in the gallery, you'll first have to delete all their uploads before you can remove the guest.