A character with this ability would be able to lock or block anything. For example, they could lock a door, window or passageway, making it impossible for any to pass through. They could also lock files so that they would be inaccessible, or even lock information so that it'd be impossible to access in any way. It could also possibly be used to occlude a wall so that it couldn't be phased through, or occlude a room so that none could teleport into it. The ability can sometimes counter the ability of access, but sometimes the opposite is true and access can negate it, depending on the strength of the 2 who possess the abilities.