A redditor who has not been successful with his last posts and seeks to downvote every possible upload. They:
-Point out reposts
-Correct Grammar
-Downvote a user's past uploads
I started out strong on reddit with a solid 30 upvotes on my post, but that karma block pointed out that it was a repost :/.
The act of not upvoting for an arbitrary reason and declaring it to the world. Typically results in no praise and the karma blocker looking like an asshole.
Jack: "I would upvote, but the counter is already at 69, hahaha"
Bob: "Hey man, stop Karma Blocking and just give them their upvote"