As far as I know there's no way to disable them. If I could, I sure would, because the way some NPC's chat is often in third person which is weird, but why do they have to do it in public is another thing. From an NPC's "roleplay" perspective *cough* those messages should stay inside the Beluga Liner. Everybody's so creepy and talking to themselves in public... [where is it]
Lol, I usually don't tend to roll down my window and say "My Dodge is in the top 1% of all trucks out there"