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1.
One who prevents or interferes with a Jehovah's witness trying to knock on someones door.
Curt placed a land mine underneath his doorstep today to knock block the Jehovah's witness.

Chris answered his door naked to greet the Jehovah's witness which caused the unwanted Jehovah's to flee his property. Chris; is the ultimate Knock Block.

Jehovah's Marty- "Lets go spread the word of our lord."
Jehovah's Pete- "Ok"
Jehovah's Marty- "OMG PETE... wait!, your leg is caught in a bear trap."
Jehovah's Pete-"We just got Knock Blocked"
by TI7 November 29, 2010
 
2.
term for someone who tries to talk people out of fighting
Jimmy: Dude they were totally about to fight but Tim had to go be the good guy and convince them it wasn't worth it.

Allen: Man, what a knock-block!!
by indiansrf March 25, 2010
 
3.
Punishment, in the form of a hunk of plastic that the band director bangs on with his conducting stick to keep time when the band keeps messing up the rhythm of a certain song. Usually makes an agonizingly sharp and loud sound, particularly for those poor suckers (like me) who are sitting in the front row of chairs.
Band director: " You guys keep screwing up this song! Joe, would you go out to my car and grab my knock block for me?"
Band members: "AAAGH! NOOOOOOOOOOO!!!"
by Blinx-the-kitty August 14, 2008