Amanda Bynes -- Fake Instagram Account Closed ... After Celebs Fire Back

Amanda Bynes did NOT rip celebs on Instagram ... it was an impostor all along ... and the fake account has been shut down.

An account called Amandabynesfashion has been populated with posts attacking various celebs ... accusing one celeb of having an STD and another of having a small penis.

A lawyer for one of the celebs who was attacked called Amanda's lawyer, and we're told her lawyer presented evidence Amanda was not doing the posting. The lawyer for the celeb -- armed with the evidence -- went to Instagram, which then shut down the account.