I hate blocking bets

[gabbahh]

If I'd be your opponent and the third spade came on the board:
If you check I will bet.
If you raise I will reraise.

So I think against your average TAG it is better to check.

Against a passive player I think you can safely check the river and fold to his raise if it is big. He's passive so he will only bet strong with the nuts. If he bets weak (indicating a low flush) you can reraise, but only if the bloke is capable of folding a strong hand. Guys who call pot size raises with a low flush draw AND fold to a reraise on the river when they made their hands are very rare.