Or you know. Equal and opposite reaction. Whatever material you make the laser gun out of that can sustain that heat at its most potent and concentrated (the gun barrel) you just... Coat your ships in. And now enemy lasers are useless. And that is completely ignoring conservation of energy. :hehe

Also, to explain the formula in layman's terms: The further the laser travels, the more enegy will dissipate into surrounding space, since it is a weapon of pure energy, this means it has a restrictive maximum range. 

Oh, and this is ignoring the fact that if you can build up that much energy, contain it, and fire it, you are equally capable of simply deflecting it. :p