The international human rights organisation said in a statement that the 37-year-old pineapple farmer had successfully mass mobilised local communities to stop the construction of the Bo Nok and Hin Krud coal-fired power plant projects in Prachuap Khiri Khan.
He was shot as he got off a bus at the Bo Nok bus station in Muang District on June 21, 2004.
He had just returned from testifying before a parliamentary anti-corruption committee in Bangkok, and had earlier registered complaints about official collusion in the illegal transfer of public land in Bo Nok.
On Tuesday, the Supreme Court dismissed charges against three members of the Hinkaew family – Thanu Hinkaew, Manote Hinkaew and Juea Hinkaew – saying the evidence against them was too weak to convict them of involvement in the contract killing.
The court affirmed the earlier Appeals Court ruling.