If you just want to remove a person's name, click on their name, click "Edit [Name]'s Details" and delete everything from the name friends.
To delete the entire person from the tree, click on the person, click "More Actions..." and click that last button that says "Delete [Person's Name]"
If you do not see the Delete button and instead see a message that this person cannot be deleted, then you have to first delete either all of their direct ancestors or all of their descendants first, so that there is no gap in the tree.
