Sorry to hear about this NyBoy, robbers cannot be a fun experience (especially when the robber can be somebody that you know and come into contact to often)!
Do you know any big, muscular guys that you can hire to actively visit your property? That might work OK as a sort of preventative measure. It would be even better if you could get him to lift weights/use punching bag/etc. I guess that there would still be a chance of that person being the perp. that invaded your privacy in the first place though. Just sort of thinking out load and trying to make a good suggestion that nobody else made yet (pepper-spray is definitely a good option). Keep an eye out for suspicious behavior coming from your co-workers, clients, etc - don't be too leery (right word?) though, that's not good for anybody's health.
Hopefully you'll get things sorted out soon.