At the moment you cannot deactivate a record from a form if any field set as mandatory is blank, system will ask you to fill in the mandatory data , save and then deactivate the record. But you can bypass this situation by going to a view or a grid showing this record, selecting the row where the record appears and deactivate it.

Ideally should have the same behaviour, and we think that users should be able to deactivate records no matter what information is missing on the form