Someone else will undoubtedly reply and be able to be more specific, but in Virginia you can use force (up to and including deadly force) if you are in reasonable fear for your own life OR the life of another (as long as you weren't part of the inital confrontation, in which case you still can use force, but under considerably more stringent conditions).
However, there is no specified right to use any degree of force to protect property - including your own. Of course, there are examples of folks who have done so without prosecution due to local mores, Virginia's appointed judges, etc. But I wouldn't count my freedom onthis factin any way.
If the robber was pointing a gun at the clerk (real or fake, loaded or unloaded - as long as a reasonable person would think her life was at risk)then you could use force to stop him. If he was walking away after having robbed her with a gun, and now was just leaving with his loot, then you shouldn't.
The General Assembly last year failed to pass a law specifically protecting those who lawfully use force, with the main argument being it is unnecesary as traditionally Virginians haven't had to worry about the police, local prosecutors, or the courts when having done so.There is no dramatic example of it having happened here. Expect another attempt to pass this bill next session of the General Assembly.