How to stop helicopters when they steal crates?

I've been playing since around christmas. Just now got the warehouse and office after grinding hard on the last 2x $ and RP. My crate missions have been going well so far. But every 3rd one or so, a helicopter shows up to attack or steal my crates. So far i have been unsuccessful in stopping them. The first time 3 copters came and i got semi close, tried to shoot them with rockets and missed. 2nd time it was a valkyrie, and i never even got close. How should i handle these?