Justin Bieber: Hotel to Mayor: Screw You, We're Keeping Bieb's Art

The Australian hotel tagged by Justin Bieber is going to war against the city's mayor ... defying hizzoner's order to remove it, and going so far as to PROTECT it from being cleaned up. 
TMZ broke the story ... Justin covered a wall at the QT Gold Coast with some colorful graffiti monsters -- angering Mayor Tom Tate, who then sent a videotaped message to the "pop princess" demanding he clean it up. 
Justin hasn't responded to the mayor, but now hotel officials have -- telling us they're so proud of the Bieb's handiwork ... they're covering it with a black tarp so no one can remove it.

Officially, we're told the cover-up is there to shield the graffiti from rain and sun ... but clearly it also 
sends a message to roving do-gooders: HANDS OFF BIEBER'S MASTERPIECE.