You can give your customers the ability to hide individual files in Presentation Mode.
In the gallery settings, please activate the functions "Allow switch to Edit Mode" and "Do not show red flagged files in Presentation Mode".
When visiting the gallery, your customers can now switch from Presentation Mode to Edit Mode by clicking the button at the top. Adding the red color flag to a file in Edit Mode will automatically hide those images Presentation Mode.
If you want, you can change the name of the red color flag, e.g. to "hide", so that it is even clearer for your customers what adding the color flag will do.