you should "remember" which buttons were are unclicked before you disable the whole board and then re-enable them when its this players turn..
it really depends on your implementation.. if u decide to extend button then add a boolean field "clicked" and then iterate over the 9 buttons and enable only the ones with clicked=false.
another option is just save the button numbers which have been clicked if you are using a grid or some kind of collection to store them.
again, im not sure what u did because u didnt post any code but i advise u to think it over and decide how do u represent a tic-tac-toe button in your app.