Last summer I had ants get into three of my cameras, it was a bad ant year. I cleaned them out the best I could and sprayed them down with ant killer. The ant killer seemed to stay with them and the ants did not come back, it seemed to have a residual effect. I can still smell the ant killer in one of them.

As a preventative matter I guess you could spray the outside of the case with ant killer.

The next best option is to make sure you sleep with the lights on! Just Kidding