First, everyone needs to know that if you hold your shares in an IRA, your shares can't be borrowed.
Second, if your shares are held in a cash trading account, your shares can't be borrowed.
Third, if your shares are held in a margin trading account, your shares CAN be borrowed.
To disable your shares from being borrowed in a margin account, you must notify your borker in writing stating the you do not want your shares made available to be borrowed.
This information is straight from Ameritrade. If your broker is someone else, your milage may vary.
GLTA