how can I remove a scratch mark most probably caused by someone using a key to scratch up my door?
The scratch is deep enough so that it can be felt, maybe passed the clearcoat and just up to the paint.... hopefully not passed through to the metal.

 

I hate to say it, but you'll need to get it re-painted. I got keyed a few months back, and all the buffing in the world wouldn't fix it. I filed a claim with insurance and had it re-painted.

 

You've got to get it repainted. If it's not too large of an area, it may not be worth calling your insurance for it. Once the metal is exposed, there's no way of restoring it without a repaint.