Here in Oz "knock off" means to steal - could be a physical object or an idea (see Mav's previous "cheap copy" definition) , as well as his first meaning of finishing work, and also to defeat esp in a sporting competition.

"Knock it off" means "give it a rest", cease and desist.

A knocker is a person who constantly criticizes (knocks) someone or something, as well as the banging thingimmy on a front door for knocking up (waking, not impregnating!) the residents.