Jeff Ross -- We're Gonna Destroy Justin Bieber ... No Matter How Much He Begs

Justin Bieber's heartfelt video about changing his ways might melt the hearts of Beliebers, but comedian Jeff Ross could care less -- he's still gonna rip him to shreds at the roast.

The Roastmaster General was just getting back to L.A. -- fresh off roasting Terry Bradshaw at the Super Bowl -- when we asked him about Justin begging for forgiveness for all his d-bag antics.

Let's just say Jeff seems unmoved.

As for what's off limits during Bieber's upcoming Comedy Central Roast ... Ross had a pretty surprising response -- especially considering JB's checkered past.

Watch this ... you can almost see Ross licking his chops.