Justin Bieber -- Double Cup Cover-Up in Calabasas

Justin Bieber's party pals are trying to hide evidence of their sizzurp abuse ... and TMZ has the photo evidence right in Bieber's backyard.

As we first reported ... the skate ramp at Justin's place was tagged with a styrofoam cup and the words "double cup" -- a clear reference to Bieber's use of sizzurp, aka lean ... a mixture of codeine cough syrup and Sprite.

But now the double cup painting is GONE ... almost.

The pic of the skate ramp (above) was taken yesterday -- and "double cup" is nearly covered completely by a skateboard someone placed on the ramp.

Compare it to the pic taken last month (below) ... where the words are bold and obvious.

Nice try guys.