Unfortunately you cannot stop your gerbils from doing this. I have had three gerbils in the past and they really are hyperactive little animals. They dig and move things around all of the time which makes them incredibly fun to watch. The problem is that it sounds like you have a wire cage. This is great for hamsters and rats, but generally gerbils are better off in a tank.

If you kept your gerbils in the tank they would not be able to throw straw all over the floor and they would be happy enough digging and chucking things everywhere! So if you haven't already, go out and buy a tank for them. They even love peat which you can purchase from all good garden stores. They can dig and create tunnels in it and it is also interesting to watch them running through them.

 I agree a tank/ aquarium will do that trick i know from experience! they will always throw thw straw cos they love digging into corners lol.funny litle things, mine wouldn't stop breeding every 2 weeks both females would pop out 8 babies each! the male was proud lol.