Invest in a set of maglocks (probably spelt wrong). They fit onto the doors and look like alarm sensors you find on windows and the like. The only difference is that they "lock" together with a magnetic force capable of withstanding 1000lb of force before they give. They work off a normal house alarm and whenever the alarm is set, they are set.

A neighbour has them on his garage, and there is no way you are opening those doors if they are active. You'd pull the garage away and leave the maglocks there