Make them fail to work if pointed upwards, at more than (say) 60 degrees.
Twats can still cause other harm with them, just as now, but accurately pointing them at an aircraft in flight would be a lot harder at a maximum angle like that.
Hmm. Scrap that idea, then - just read a little further and saw the high rise blocks/Glasgow airport comment. A low angle would probably still be a problem there. Reduce the strength of the laser as the angle increases, so at anything above horizontal, it's too weak to do any harm.