Unless he has a no trade clause in his contract, he can't block a trade. Since he was traded from the 50-win Nuggets to the 23-win Clippers for a 2nd round pick and didn't block that trade, I'd say it's likely he doesn't have a no trade clause. There are VERY few players in the league that do. Kobe is one. Not sure if there are others, but Marcus Camby would not have been a big enough star to have the leverage to get such a stipulation built into his last contract.
BNM