Cash accounts appear to be immune to shorting if you set your price and lock them. However, it seems like many brokerage firms in their Margin agreements have the ability to lend out.
No offense to anyone. 9 times out of 10, the people that want you to lock your shares high are selling below you. I always set my limit order at a reasonable price I am willing to sell at if the stock runs. If my order fills and the pps keeps rising, I am still green and that is always a good thing. Trading 101.