When Guest Users are disabled from the backend to use the Remove Background feature, they cannot apply that feature to the images they upload onto their products.
Login to Backend
Navigate to Stores -> Configuration.
You will be directed to the Store -> Configuration page. Go to Brush Your Ideas and in that, click on Product Designer Settings. Scroll down to Remove Background.
Go to Enable Remove Background For Guest User and select “No” option.
This will restrict the Guest Users to use the Remove Background feature.