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 him, including Walkers, Builders or Miners, will turn around and start going in the opposite direction. A blocker can become a normal Lemming if the ground beneath him is removed. More often, they are removed using an exploder, or nuked at the end of the level. In The Lemmings Chronicles, blockers perform a similar role, but can be freed at any time using the Walker skill.
Trivia
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.