Blocker is a skill that can be given to a lemming. It is the fourth of the eight skills available in Lemmings, as well as the Classic Tribe in Lemmings 2. A blocker stands still, and any lemmings coming in contact with its hands, including walkers, builders or miners, will turn around and start going in the opposite direction. Lemmings that fall on top of a blocker will walk past it going their original direction. A blocker can revert to a walker if the ground beneath it is removed. More often, they are removed using a bomber, or nuked at the end of the level. In The Lemmings Chronicles and Lemmings: The Puzzle Adventure for mobile devices, blockers perform a similar role, but can be freely reverted to walkers at any time.
In Lemmings 2, blockers are only available in the Classic tribe. If used in any other tribe (achieved by hex-editing a level file, for example), they will stand still but other lemmings will walk past them.