Justin Bieber Says Too Bad, Selena ... It IS Too Late to Say Sorry

Justin Bieber won't give Selena Gomez the time of day, or a chance to formally apologize for publicly bashing his current relationship and insinuating he cheated during theirs.

JB sources tell TMZ ... Selena's been trying to contact Justin since the social media war went nuclear Monday night -- but Justin's not responding. We're told he feels like his ex won't let go, and she's constantly looking for ways to insert herself in his biz.

Bieber's particularly amused Selena ripped him for threatening to quit Instagram, because she did the same exact thing earlier this year. We're told he thinks it's pretty two-faced and, right now at least, he's "very over her" ... even though she sort-of apologized on her Snapchat, admitting what she said was "selfish and pointless."