Brittny Gastineau's Ex-BF -- Street Artist Retna Arrested for Pap Slapping ... Pulls a Reese with Cops

Brittny Gastineau's ex boyfriend was hauled off in cuffs while doing his best Reese Witherspoon impression ... after a fight with a photographer ended with a blast of pepper spray outside Craig's.

Street artist Retna -- who had a brutal fight with Gastineau last year -- was leaving the West Hollywood restaurant Monday night when he got into an argument with an independent paparazzo ... not one of our guys.

Witnesses say Retna appeared to have been drinking, and got in the camera guy's face. When the pap complained about Retna getting in his "personal space" -- Retna allegedly slapped the guy ... who retaliated by firing a can of pepper spray.

Law enforcement sources confirm he was pretty wasted, and say he was booked for misdemeanor battery.

You gotta see Retna getting stuffed in the patrol car, and yelling things like -- "I'm a U.S. government!" Reese did it better.

Easy to see why Brittny's moved on ... and is now rumored to be seeing ex-bf, singer Luis Miguel again.