There is such a rule, though it is broader that your specific hypothetical; conduct detrimental to the League.
Forcing him to sell is actual doing him a favor, believe it or not. If they said, "Keep your team Donald, we're just not going to schedule any of our teams to play yours. Have a nice life." just how much value would you imagine the standalone LA Clippers to have going forward? Aside from the physical property the team owned and any merchandising the team could still muster, not very much. A sports team is not a standalone entity, in terms of it's value. That value is, largely, based upon it's relationship to the other franchises. Requiring him to sell it while maintaining that relationship preserves it's value for him to capitalize on.