After two unexpected upshots, newly crowned Thai League 1 champions Buriram United finally succeeded in putting some icing on the cake with a 4-0 home victory over nine-man Nakhon Ratchasima on Friday night.
The result condemned the Swat Cats to at least a year in the second tier.
After Goran Causic had given the Thunder Castle the upper hand in the 13th minute, the visitors were reduced to 10 men in the first-half injury time when Jordan Murray was red-carded.
Supachai Chaided (60 and 90) and Peeradon Chamratsamee (66) struck in the second half and the Swat Cats lost another player, Amani Aguinaldo, to red card in the 78th minute.
The three teams to have been relegated this season are Nakhon Ratchasima, Nongbua Pitchaya and Lampang.
In a major final day shocker, Lamphun Warriors defeated third-placed Port 3-1. Elsewhere, Bangkok United were held 1-1 by Khon Kaen United, BG Pathum United edged Chonburi 1-0, Muang Thong United routed Ratchaburi 4-0, Police Tero upset Chiang Rai United 2-1, PT Prachuap beat Sukhothai 2-0, and Nongbua defeated Lampang 1-0.