The same section of the NBA Constitution which gives the Commissioner the power to sanction owners provides that the Commissioner's decision is final and cannot be reviewed in court. That would probably (but not definitely) be upheld if Sterling challenged it.
If I remember contract clauses seeking to oust courts of jurisdiction are illegal. There could be an agreement to arbitrate before any filing in court or in lieu of judicial action. To put it another way arbitration clauses are legal. An arbitration would probably not do him any good. Secondly, not withstanding any contract provision to the contrary one can always raise constitutional issues. There have been lots of penalty clauses in contracts voided as unconscionable even under ordinary case law.