This is currently not possible. A workaround is to create a second attribute and enumeration that contains only those values you would like to show in the dropdown. Then, use microflows to synchronize the values of the two attributes, e.g., onChange of the enumeration dropdown.
You could also choose to go for referenced objects that contain the values you would normaly put in the enumeration. It will then be possible to constraint the options you show to the user simply by stating [not(Objectname/AttributeName = 'ValueYouDoNotWantToBeShown']