L.M.
I have boys 13 and 11 and have already had my fill of fighting having them home for the summer all day. I give them warnings, but when they get physical I take away all their privledges and only allow them "to play the PS2, but only a 2-player game that they must share" or they cannot play......at first my oldest said forget it and I told him option #2 was to weed my garden bed together. I had peace all afternoon and it has definitely been a lot more quiet. Sometimes you almost have to pull a nutty on them to make them realize that you have had enough. I also explained to them in a calm voice that it is not pleasant to be around people who argue all the time and that they might want to consider that since it would begin to effect friends coming over. Now when they are fighting I just say "that doesn't sound like care or sharing words.....do you two need some more time sharing" and it magically stops.
Good luck