I'm having a similar problem and almost made a thread asking for help.
I have two elderly cats and one young one that I kept after he was abandoned by my roommate. The young one, Chico, wants to play all the time, but the older two, Gizmo and Tyler, just want to sleep. Also Gizmo has arthritis and doesn't get around so well. Chico will go over and groom Gizmo and won't stop until Gizmo is growling and biting at him, and finally runs away. At night he dominates him and won't let go until I get up and chase him off, Gizmo isn't strong enough to get him off himself. He tried to get Tyler to play and doesn't stop until there is a fight. I'm at the point of asking friends if anyone wants Chico. It isn't fair that the older cats are constantly harassed, or that Chico gets in trouble for wanting to play. Plus I'm exhausted from getting up EVERY night around 3 or 4 to get Chico off Gizmo, sometimes twice a night.
Anyone have any suggestions on how to stop this behavior? I don't have anyway to keep them separate.