Both Ameritrade and E-trade require that you call them and request that your shares are not to be loaned to the shorts. Merely placing a limit sell order - AON - GTC - doesn't accomplish what you want it to.
Anyone here can call customer service at either Ameritrade or E-Trade to verify what I've posted.
I think it's somewhat irrelevant with $AMBS as neither allows $AMBS to be shorted. It may be relevant to other stocks you own or when $AMBS up-lists.