I haven't met a single Kahn online yet, so I'm not really qualified to comment on him, so I'll move on.
Amazingly, the only flash I've met online didn't spam. He didn't win any of the rounds either, so...
I've fought a few Scorpion and I've only played one match where I didn't play as him, so I feel qualified to talk about him. If you're standing still and blocking a lot, hellfire is in your future. Scorpion will use it for easy extra damage, some will follow up with spear, but you get time to block it. Teleports can be blocked, though a good Scorpion will probably use the aerial version, which is quicker, or incorporate the standing version into a combo. If you're standing a blocking a lot while he's on the offensive, he may try to slide you, since the slide hits low. Finally, if both players are keeping their distance and walking around in circles, he'll probably wait for you to do a special so he can; a) throw spear and air juggle you,

teleport and air juggle you, or c) play it safe and hellfire you. After walking around in circles for a while, I've just taken the initiative with a full charge ending in a slide, usually catches them off guard.
Raiden - Raiden is the only character I consistently lose to, mostly because there's a 0,1 second delay and I can't properly get my blocks in after he teleports. However, if you see it coming, there are a few ways around it. Personally, if I'm in the air, I teleport, but not a lot of characters have aerial teleports. If I'm on the ground, projectile or slides should suffice, though there's a chance with a slide that you'll slide in the wrong direction and end up vulnerable.
Superman spamming ground pound... again, I haven't encountered any Superman players who spam ground pound yet. In fact, only one player has ever used the move. I've seen a few solutions, Rage breaks it. Teleporting might help some characters. Projectiles, like Sub-Zero's freeze should work as long as you can get it off before the actual pound. Being Superman and going completely aerial on him should force him to give it up. If all else fail, wait til round 2 and spam him with your annoying moves. If he thinks his ground pounds are annoying, just wait until I start my aerial teleport spamming.
I'd just like to add Sub-Zero to this. He's not really annoying, but I've fought him a lot. For some reason, half the players I fight feel the need to have Scorpion vs. Sub-Zero fights when I go with Scorpion. Anyway, he'll do that freezing himself move a lot, especially when you move near. If you don't touch him, you'll be fine. If I'm not mid combo, which means I'm screwed anyway, I'll hit him with a hellfire. If he teleports near you, he'll probably freeze himself again right afterwards since his teleport is kinda slow. Again, I just hit him with a hellfire or wait until it starts wearing off before I hit him. Some Subbies will start just spamming 4 when the get close, a little lag and this can get you into serious trouble. Just block and counter with a fast move or an uppercut.