Play video content TMZSports.com

Ex-NBA star Gilbert Arenas took a cinder block and smashed the hell out of a Mercedes that his longtime GF Laura Govan had been driving ... and he claims the whole thing was revenge for Laura ruining his Netflix account.

Sources connected to the situation tell us the incident went down in 2014 ... when Arenas was so upset, he decided to take his anger out on the luxury vehicle in the driveway of his home.

Gilbert says he has a perfectly good explanation -- claiming Laura had thrown his computer in his pool and smashed 34 windows at his Calabasas home.

Gilbert says, "I don't remember my #Netflix password it was auto saved, you piece of sh*t. u take a person #Netflix away,nothing else matters."