The main way to block your plate is with a bike rack. A guy in Calgary was ticketed for it so he went to court to fight it. He took a picture of the city's bike patrol van with their rack covering the plate. The judge laughed, and  said the police were exempt, but let him off anyway. 
What I find amusing is senior citizens (who rarely speed)are obviously defacing their plates so they can't be read.  Another is a guy with a vanity plate that looks like he took a grinder to it. Why spend $150(or whatever) then deface it so it can't be read? You can be fined and forced to replace it.