I've been reading "Small Stakes Hold'em", focusing on improving my post-flop play. I'm trying to figure out how to properly protect a dominant but vulnerable hand against draws, and doing the math, it seems in most cases you can't really make it unprofitable for opponents not to draw.

For example, say I flop top pair or two-pair, a hand I'm sure will win if it isn't outdrawn. There are 6 players, and the the pot has 6 bets in it. Say two spades came on the flop, and I'm pretty sure one of my opponents has two spades and is on a flush draw. I know I need to aggressively protect my hand, but, the fact of the matter is the pot already has 6 bets in it. If it's a bet to the opponent on the flush draw, he has a 1 in 5 chance of hitting his flush on the turn and winning x6 bets - so calling is the right decision for him at this point. Now, I could raise him, and say that convinces two players after me to fold, but the rest call, so now there are 4 players total remaining. The pot now has 13 bets in it, and it's back to him. Now he must pay 1 bet for the chance to win x13, with a 1 in 5 chance of hitting his draw on the river. Still a profitable decision for him; me raising did nothing to make calling unprofitable for him.

So what am I missing? When is raising to protect your hand against draws effective?