I NEVER advise PUNCHING & KICKING BAGS to RELIEVE FRUSTRATION or ANGER!
When we are angry and we hit to get rid of the anger, we are conditioning our self to "hit when we are angry to get rid of the anger"! This can program us to "want" more anger because it is making us "better" at punching & kicking. Eventually hitting out of anger can become an instinctual response regardless of if it's a bag or person standing in front of us.
Getting rid of anger is about learning to understand "why" we are angry, "what" actions lead us to become angry, and "how" we may respond when angry. Then we practice controlling the conditions and correcting our emotional responses.
Martial Arts & combat training IS NOT for releasing anger. It is for learning to control our responses when emotional. The challenges of training and sparring under controlled, safe conditions allows us to tap into these peaked emotional moments where we usually would shut down, explode, cry, or run. Then while peaked and not knowing how to "act right", the instructor should be there to guide a new, productive mindset and behavior.
Oh! And the way to relieve frustration, anger, and stress is with meditation, stretching, Katas, Shadowboxing and discovering the root of these problems to address them calmly and constructively.