Air carrier Captain reported they had to brake hard and make a sudden stop to avoid a vehicle crossing their path.

Narrative

Our aircraft was cutoff by a tug driver nearly causing a collision. Our taxi route was Taxiway 1-Taxiway 2; to Taxiway 3. While joining Taxiway 2; and approximately 50' from the VSR (Vehicle Service Road); a tug was approaching the stop sign on the east of the service road; did not obey the stop sign and continued driving in front of the approaching aircraft. To prevent a collision; I had to step hard on the brakes and brought the aircraft to a full stop. The incident occurred at approximately XA:10 local time and was reported to ramp control.To prevent a collision; an incident or accident vehicles in the non movement area need to obey local traffic rules; including; but not limited to; obeying stop signs; speed limits and respecting the right of way of an aircraft.