12:52 PM PT -- A spokesperson for the TD Garden tells TMZ Sports Arnold is now banned FOR LIFE for allegedly tossing the can onto the court.
"This act is a serious violation of our guest code of conduct," the spokesperson said.
Police say they've arrested a man who allegedly heaved his drink onto the court during the Celtics vs. Spurs game Wednesday ... identifying the guy as a 22-year-old and hitting him with disturbing a public assembly charge.
Late in the third quarter of the Celtics' game at TD Garden .