JUSTIN BIEBER I Didn't Spit on Him ... I Didn't Threaten to Kill Him

Justin Bieber is adamant his neighbor is LYING -- insisting he NEVER spat or threatened to kill anyone during a heated exchange at his home in Calabasas ... sources close to the singer tell TMZ. 

Our sources tell us ... the singer isn't denying that he confronted his neighbor when the guy stormed on to his property early Tuesday morning ... but Bieber is saying things never turned physical.  

As we first reported, the neighbor was steaming mad when he confronted Bieber ... and yelled at Justin for racing his Ferrari around the private community at breakneck speeds. 

We're told Justin believes he had a right to engage the guy in the argument ... but the singer is telling people no spit ever left his mouth. JB also insists he never threatened to do the neighbor any harm. 

Plus, our sources tell us ... Justin's camp believes the neighbor was trespassing during the entire incident.