i can't tell you exactly what to do but I'll give you the ingredients you need: it will envolve a fire hydrant, the anus, a bottle of tobasco sauce, and a north Arabian tree frog that has trust issues ever since his parents left him.
Is he trying to get into the room or does he just like to scratch?
If he's trying to get in, let him in. If you don't want him in the room, ignore the scratching and he will eventually see that...
Use a Dogpile.com search and they have high dollar static electric mats , or if the cats really tick you off, sneak up on it and whack that cat hard off the car, or if it is a hard top and you are...
If your feeding the stray cat then its more then likely to keep coming there
so IF you are feeding it Stop don't throw scraps out that your cats wont eat
and take notice of how much your cats eat...