Take a look at the XPath constraint (Constrained by) functionality for reference selectors. With it you can constraint the possible options in the dropdown. You can find it under seletable objects if you select Xpath.
For example if you have Classes and Students, and you would only like the student to select classes which he has not selected already:
Hope this helps,
You can make the source for the dropdown menu based on a microflow in which you retrieve the list of values which hasn't been added yet.