You have to be careful about deleting people from Ancestry trees - you have to do it 'from the ground up'.
For example, if you had John Smith with three sons Tom, Dick & Harry, and you delete John Smith, you will leave Tom, Dick, and Harry floating about in the ether, and they won't show up in your tree. Although this is not a big deal, this could cause problems if you export the GEDCOM file to use in another genealogy program, and all these 'orphans' will bloat your tree.