What are some strategies for killing a super fed champion that is completely running the game?
It really depends on who it is that is fed.
Normally, the most viable way to keep them under wraps so they don't completely dominate a game is to stick together and focus that person in a team fight.
Otherwise, just build to counter them. For instance, a few months back I played Kog'Maw with an on-hit magic effect build: Madred's Bloodrazor, Malady, Wit's End, etc. I was like 11/2 before the other team wised up and purchased so much magic resistance that my build became irrelevant and I couldnt do anything to any of them which ended up losing us the game.
So counterbuild, and focus that person with your team. Those are probably your two best options, in my opinion.
Thornmail is not a good item. You're better off getting frozen heart or randuins omen. Thornmail only gives armor and the passive and if you're a tank you shouldn't be focused anyway. FH and RO both weaken the ad carry and give better stats. Black cleaver and Abyssal Scepter are both just meh for arm/spell pen. Last Whisper and Void Staff are way better for the pen.
Get your team to work together and try to catch them alone. Short of that, try to focus them in the team fights so that they'll be down for a while so your team can clear other objectives without being harried.
Try to build items that can counter-act the enemy, like a Black Cleaver or Abyssal Sceptre for a tank, or a Thornmail for an AD Carry, and so on.
Don't feed them :P.
But if you're in that situation where one champ is super fed, the way you counter them will depend on which champion it is. If the champion is AP based, you'll want to stack magic resist and also grab defensive items like Banshee's veil and Quicksilver Sash. If it's an AD that's giving you problems you should stack armor. If it's somebody like a Mundo or AD Sion who just doesn't die, grab an Executioner's Calling and then ignite that champion.
In order to take them down, you'll have to organize your team to focus on that champ. If it's an AD carry, that shouldn't be much trouble since you should burn them down fairly quickly, but if it's a reasonably tanky champ you'll need to grab some armor/magic pen with a Last Whisper or Void Staff respectively.