If their behavior annoys you, why not DMCA takedown the repo in retaliation for them stripping the copyright header? It'd be within your rights since they violated the license and sends a message they can't ignore.
Otherwise if you're upset they otherwise used code you released under a permissive license that's a personal problem. You signaled to the world that you'd relax your copyright under a permissive license and don't like the results? This is like gifting something and being mad about how the gift was used.
As others have pointed out you don't seem to grasp why licenses exist in the first place? No org or individual in general wants to play guessing games and risk liability. It's entirely on you to signal how you want your work to be used.
Otherwise if you're upset they otherwise used code you released under a permissive license that's a personal problem. You signaled to the world that you'd relax your copyright under a permissive license and don't like the results? This is like gifting something and being mad about how the gift was used.
As others have pointed out you don't seem to grasp why licenses exist in the first place? No org or individual in general wants to play guessing games and risk liability. It's entirely on you to signal how you want your work to be used.