There is a system out there that is very good for teaching self protection: Silva. It's undergone several name changes, but it's still basically the same program taught by Jose Silva for many years. Now it's called Silva Life Systems. It's worth the money for the home study course, or even better, take the live seminar if possible.
I have never really had any problems with psychic interference, not the type you folks are talking about anyway. I have known people to try to sabotage my success in several areas, but you really don't have to allow it. Silva has some nifty techniques to teach you how to prevent it--plus a lot of other good stuff too.
One thing I do suggest that you consider: whatever we focus on, we make stronger and get more of. (Excuse the butchered grammar.) Protect yourself from the interference, yes, but don't obsess about it.