#1 to unbind you must know the process in which it was
#3 we cannot transmute any bound
#2 you cannot forcibly bind spirits whose vessels are lost without their permission because as #2 states, you cannot FORCE a spirit to be bound.
Not only that, but conjurer/practitioners have no reason to try; they are capable of conjuring spirits so why would they want to do that when they can conjure what they want?
Binding isn't a simple thing you can do without either training or without a tool already prepared with the necessary enchantments for you to activate. And unbinding requires you to know how it was bound in order to deconstruct the binding and reverse the process. Not to mention any conjurer/practitioner worth their weight in gold will place certain enchantments on their bindings to safeguard the spirits & spells from anyone trying to be bothersome.