Resource guarding is a common issue and one that responds very well to behavior modification.  While every case is individual generally a detailed counter conditioning program is very effective.  things that do not work and can actually make the dog more dangerous are: throwing things at the dog, shouting at dog, or hitting.  Resource guarding is about fear so to truly modify the problem you need to address the fear.  An excellent resource on this topic is Jean Donaldson’s book “Mine!”.