If you select "None" from the link accounts drop down it will unlink them.
It will also unlink from all others. Would be nice to be able to unlink select individuals. Example: two families accidently get linked via insurance transfers, then to separate them I have to unlink each person from all others, then re-link to the family/group they belong to.
Don't let what you can't do keep you from doing what you can do!