Evelyn Lozada Warns Chad Johnson ... Stop Harassing Me NOW!

Evelyn Lozada fired off a cease and desist letter to ex-hubby Chad Johnson ... warning him to stop harassing her online ... OR ELSE.

Sources close to the "Basketball Wives" star tell TMZ ... Evelyn's lawyer sent the letter Thursday after Chad BLASTED her on Twitter ... making crude comments and accusing her of cheating.

Sources close to Evelyn tell us ... Chad's Twitter allegations are 100% false -- she NEVER cheated and if he doesn't stop spreading lies STAT ... she'll get a restraining order.

Not only that, we're told the prosecutor's office overseeing Chad's criminal domestic violence case is aware of the situation and is concerned. We broke the story ... Evelyn filed for divorce after Chad was arrested for allegedly headbutting her just weeks after they got hitched.