how do I detach someone from my family tree?
John W. Dougherty and Rachel Thomas are attached as "preferred" parents to my great-grandfather. I know for a fact that this is not accurate. How can I detach them?
So preferred has nothing to do with the parents that are biological. The Preferred setting tells the system which line to display in the pedigree views. Could it be that John W Dogherty and Rachel Thomas are parents of a different sort - guardian, foster, etc? I make it a point to add all the parents a child lived with while growing up, and I make sure the relationships are correct. And I set "preferred" to whichever set of parents I am researching.3
As Gail said, "preferred" has nothing much to do with anything: it's a per-user setting that defaults to whichever relationship was most recently edited when you first viewed the profile. For profiles that have more than one set of parents, or more than one spouse, you use this setting to control which relationship is shown in things like the landscape or portrait pedigree view.
The easiest way to remove incorrect parents is to go to the child's profile details page, scroll down to the Family Members section, find where the child is listed under the incorrect parents, and click the pencil icon to the right of the child's name. On the resulting popup, click "Remove or Replace" below the child, and follow the prompts.
Your correction is more likely to "stick" if you research why those incorrect parents were attached, and perhaps find their actual child or children to attach them to.3