As per 30AUG12 GDS inventory display, THAI starting 16JAN13 cancels Bangkok – Koh Samui service. The airline currently offers twice daily flights on board Boeing 737-400 since 15FEB08.

Reservation for this flight is closed and removed from the system for travel on/after 16JAN13.
Schedule until 15JAN13:

TG281 BKK0745 – 0850USM 734 D
TG287 BKK1520 – 1625USM 734 D
TG282 USM0930 – 1035BKK 734 D
TG288 USM1705 – 1810BKK 734 D

Source: airlineroute.net

Bangkok Airways is still flying to the Island and they also own the airport.