Beyond the dreaded "cert call" which severely restricts a stock's liquidity, and placing a sell order....and frankly I don't think having a GTC sell order for $20 when the PPS is less than $2 would stop shares from being loaned out.

Beyond these two there is also holding shares in a registered account.  For shares to be loaned out to short sellers they must be in a margin account.  If you have a registered account, such as with an RSP or TFSA account, then the shares are not to be loaned out in any case.

As for putting a GTC order in....one of the brokers I use won't accept a sell order at $20 on a stock trading at $2, let alone $1.50.  

However it might not matter....based on the TSX forced buy ins it looks like shorts have been selling into the market regarless of being able to locate the shares or not.