Perhaps the guys from Dimagi can express other options, but as far as i know, you cannot directly manipulate look up data from inside forms.
what you can do to achieve a similar functionality to what you’re asking however might get a little complicated.
the easiest way i can think of is to use a filter on the lookup table data with two functions, selected() and join().
basically, you want to join all the strings from all the cases that have selected a value from the lookup table using the casedb. something like “join(instance(‘casedb’)/casedb/case…casetype etc…)”, and then use not(selected(joined string “join(instance(‘casedb’)/casedb/case…casetype etc…)”, lookup field where unique value is taken from) to make sure that the options you see loaded from the lookup table have not been selected in another case. it’s a longer query, and has many potential failure points, but, might be worth a try.