When I bind X-refs can I unbind?

Quick answer = No.

If you bind the external references as an insert, you can delete the block and purge the drawing to remove the block definitions.

My first question is: Why bind?

If is to go to the client, your best bet is to e-transmit. This way, you don't have to worry about not having the correct program to bind. For example, last I recall, you cannot bind a CADworx drawing without CADWorx loaded.