I'm not sure I understand how the seating works since you don't mention seat 9 preflop but say that he plays tight at the end. In that case, I'll take it that seat 9 is actually seat 1 wich is the original UTG + 1 raiser.
With two people on board and the loose guy checking it down, it is pretty obvious, at least to me, that you should bet it strongly in order to block high draws (As,Kx, AxKs are all in tight guy range, as is AsJs or a pocket pair that contains a spade, not to mention AKs wich has you drawing dead / loose guy could have any two spades, wich still has you beat).
It is pretty much bound that somebody has a high spade or will represent a higher spade later on in the hand if they call your flop bet. So I'd bet 3/4 pot and reconsider my options later on if I'm reraised or if I get a caller and another spade hits on the turn. I don't feel that this is a hand that can give you a huge pot, but it can and probably will make you give your stack away if you cling to it.
Try to take the pot but let you got to let it go if the cards/player don't cooperate. This is what you get from playing junk hands such as 4s2s You can really and probably will, stack a player if you hit the flush on the turn/river against someone who probably has hit his set, but flopping this weak flush is hell and goes against your original move (trying to trap someone).
PS: Another way of playing it would be checking it down and trying to keep the pot small/get out of the way on subsequent streets. But the thing is that you probably have the nuts right there (and if you don't, you make a good laydown), so betting it strongly would be my first option. Maybe you'll get lucky and somebody has a weak flush and thinks yours is better...