(covers information from several alternate timelines)
A security lockout was a safety procedure that was performed when it was desired that someone should be prevented access to a computer system.
In 2367, transporter chief Hendrick locked off access to the controls in transporter room 6 aboard the USS Enterprise-D. The security lockout was later broken by Geordi La Forge who was affected by the Tarchannen III species and beamed down to the planet. (TNG: "Identity Crisis")
Criminals stranded on Mab-Bu VI that temporarily took over the bodies of Data, Miles O'Brien and Deanna Troi in 2368 used a remote security lockout to prevent the crew from shutting down all computer functions in Ten Forward where the criminals hid with their hostages. (TNG: "Power Play")
When renegade Ferengi hijacked the Enterprise-D in 2369, Commander William T. Riker ordered a security lockout of all command functions. Under the guise of teaching the Ferengi to pilot the ship, Riker instead diverted command access to Captain Jean-Luc Picard, who was being held in a classroom. (TNG: "Rascals")
In an alternate timeline created by Q, Captain Jean-Luc Picard recorded a personal log during one of his visits to the past. Not wanting to inform that crew of his experiences, he encrypted the log under security lockout Omega 327. (TNG: "All Good Things...")
In 2372, with her crew falling into a catatonic state induced by a Bothan infiltrator, Captain Kathryn Janeway of the USS Voyager initiated a security lockout of bridge controls through a verbal command to the computer, as she entered a turbolift for main engineering on deck 11. (VOY: "Persistence of Vision")
When the USS Voyager was under control of the Kazon-Nistrim in 2373, The Doctor and Lon Suder, the only crew members left on board, sabotaged the ship. After Seska discovered their work, she instructed the computer to disregard all voice commands from Starfleet personnel and put it on security lockout. (VOY: "Basics, Part II")