The only way you could prevent someone from finishing would be if you are the leeward boat and in a situation where you are not restricted by proper course limitations (i.e. someone tries to pass you to windward), you could head that person up as high as you please outside of the 3 lenght zone. You would then have to bear off extremely quickly, breaking the overlap and shooting down to the mark. Since the overlap was broken, the inside boat would not be entitled to room and you could close the door on them. Frankly, I doubt it could be pulled off on a catamaran, perhaps in a dinghy.
There are some moves that you can put on another boat to pass her before the finish. In this scenerio (downwind finish, overlapped) if L is not restricted to her proper course by rule 17, then outside the zone, she can head up and drive W to the point where they have to gybe back to the finish. If they go far enough, then L will be ahead after the gybe. If not, she'll be inside and to leeward at the finish and won't have to give W room. This tactic may even work better on catamarans, because gybing duels are typically more difficult than on monohull dinghies.
On upwind finishes, one of my favorite tricks is to get on the lead boat's weather hip on the last tack before the layline. Even though W is behind, she controls L's tack. W then drives L well past the layline, to the point where she can tack in front of L and reach to the finish. This ploy works better on cats than monohulls, because it's harder to control a boat that tacks quickly.
Regards,
Eric
