Hi Chen,
Apologies for the delay. I could have sworn that I replied here.
As far as I know, there isn’t a way to do that out of the box. That “limb” that you detach is still being controlled by the AnimBP for the character. The only way that I can think is to do a layered blend per bone in your AnimBP, but that hardly seems like a viable solution.
What is the end result that you’re looking for? Perhaps there is a better way to do it than “Break Constraint”
-Matt W.