The only truly effective way to affect the behaviors of your cat when you're not around, with the exception of pharmacodynamic agents, is through the imaginative and highly logical art of booby-trapping. You can also affect the occurrence of some cat behaviors through nutritional and energy-level management modifications as well as environmental changes. But to work on an animal's motivation to engage in a behavior directly, you must attack it head on, confronting the animal with the situation and letting him see for himself that he does not like to do it. Through booby-trapping, whenever he tries to do it, something happens which scares him so much that he does not want to attempt to do it again. For a long established behavior, he will likely need to experience this fright a few times to fully absorb the lesson.
Cats rely very heavily on their sense of sight.
It is their vision which tells them where things are and what those things are doing. And that information lets them know whether what is going on out there affects them or is of interest to them. They will really only fight back if they see or have seen the object that is a clear target to attack. They will hide for a length of time that directly parallels their level of anxiety. Don't expect that they'll venture out before they feel safe and secure enough to.