He can't fire them but he does have options.
He can sue them (he has moved slowly towards that) and he can also legally side with the state and take other actions against them.
So far, he has chosen the "rhetoric" option along with moving slowly (but talking about) option of lawsuits and has avoided any other actions that he thinks would be harsh.
FWIT, Pataki actually does hold all the cards, and hasn't played them, you know its bad when Elliot Spitzer is looking more willing to hit the union and doing all the legal heavy lifting.
Granted, Spitzer is a political oppurtunist, but he sees an oppurtunity here, and he is trying to move in on it, on the side of the people and against the unions (who have never liked him anyway).