you can put in a GTC order at a high price to where it wont fill and this is said to lock-up shares so brokers cannot use them to short...i have, however, heard a lot of debate over whether this actually works...i used to do it but people who know a lot more than me said it doesnt' do anything. the only way i've heard to 100% lock-up your shares it to pay to have the certificates physically sent to you.