Spencer Pratt Gets Whipped Hard in Jiu-Jitsu Class

Spencer Pratt's haters -- and there are many -- are gonna LOVE this clip.

Heidi Montag's hubby just earned his brown belt at Paragon Brazilian Jiu-Jitsu in the Valley ... which means he's now a certified badass. He's one step away from a black belt, after all.

But the former 'Hills' star celebrated his martial arts achievement by taking a thrashing from everyone in his dojo ... even little kids.