Egg is right. The only way to ensure that your shares will not be lent out is to move your shares into a cash account. I don't know why the customer service reps don't give a straight answer on this. If you ask to speak to someone in the lending room they will tell you that the shares may be lent out if held in a margin account.
Of course, there's nothing wrong with setting a $300 GTC…unless the buyout comes in at $350!