Anime Studio Pro 11 Frezzing Layers so its not affect by bon

[sillyanimate]

Does anyone knows how to freeze layers so that its not affected by a bone after a certain frame?
In my animation, I want my character to drop a piece of chalk, first I bind the chalk layer to the arm bone....so it automatically follows the arm where every it goes.
then I went to the frame where the chalk is drop, then I manually move the chalk to the ground creating a key frame. but next I want my character to move there hand but the layer is still affect by the bones its bind to... Help?

[synthsin75]

Bind it to an extra bone and use animated bone parenting to unparent it from the arm bone.