No, we ignore it because while some of your spades may have been dealt, so have other cards.

If the cards that have been dealt are proportionally distributed in suits, then the proportion of the spades left in the deck should be the same.

So if 10 hands are dealt, 20 cards, there would be, on average, 5 spades there. But if you're going to remove those five spades from the calculation, then you should not count the probability of drawing one of the remaining 6 spades (two on the table, five dealt) out of 52 cards, but out of 52-20-3 cards, which is roughly the same (~20%).

Hope that helps.

Cheers,
FP

Edit: But there are times when you must start to figure that some of your outs are "gone": If you have read opponents as holding cards that you are looking for. Say you have AhKh, and the board is Kd-Ks-8s-3s and two people are raising, and you read them to have a flush or a flush draw. In this situation, you might want to figure one of your outs to have been dealt (the As), and you're therefore drawing to a (maximum) three-outer (Ad, Ac, Kc), not four.

Edit 2: Before someone points out that there are more outs than three (because the board can pair up again) - that was not the point I was trying to make. Just that a read can reveal that some of the cards you're hoping to catch are likely held by another player.