You cannot remove a member from wikitree, but you are able to detach a member from his immediate family by clicking "edit" in the person's toolbar, then down the right hand side of the edited page, you have the facility to edit/remove the father & mother. Remove each parent seperately, and/or spouse, saving each action carried out.
When complted, the person should now be a "stand alone" profile. Find the correct tree and then add the "Stand alone", as a child to parents, siblings or spouse by using the members ID.