I appears the Thule no longer ships the pad-lock security device for the hitch. They now offer the Snug Tight Lock cylinder for securty. Combines the Thule anti-wobble fastener system with locking bolt hear protector. http://www.backcountry.com/store/THU0186/Thule-Snug-Tite-Lock.html

I have a 3/4 bolt that screws into the welded nut inside the hitch tube, un the bolt I have slide a metal washer that has a c-shape the allows me to use a standard masterlock so people cannot access the head of the bolt. This came stock with my rack, and I am unsure if you can get one aftermarket. My brother lost his lock washer and had the welded nut removed so he could use any brank locking hitch pin. This worked good but the rack wiggles more because it is not bolted down.