If you have a person with a spouse but also an "unknown spouse", here is how to get rid of the unknown spouse:
- Switch to the Family View
- Bring up the person and unknown spouse in the family view (the spouse will actually just be blank)
- Highlight the person, then do Edit > Delete > Family from the menu
- Choose the default option, which is to unlink the family members from each other
This simply unlinks the person from that unknown spouse, but doesn't remove the person from the database.