11/20 -- The NBA has fined Kevin Durant $25,000 for "directing inappropriate language toward a fan"
2:38 PM PT -- Another witness tells TMZ Sports ... the hecklers had been bombarding KD with the "cupcake" insults to the point where it became unbearable.
That witness tells us KD complained to the refs at one point but the fans were not removed or disciplined in any way.
The witness says KD's explicit outburst was completely justified. Kevin Durant went off on a heckler who was sitting courtside at the Mavs game this weekend -- telling the guy, "Watch the f**king game and shut the f**k up.