How will a satellite powered by RTG be terminated at the end of its life? Is there any set of rules to be followed? What's the general procedure?
Viking 1 was inadvertently terminated by overwriting the antenna pointing software. Contact was never regained.
Viking 2's rechargeable battery died, ending that mission.
So neither Viking lander was deliberately terminated.
The Galileo orbiter was sent into Jupiter's atmosphere, where it was destroyed.
Cassini will be sent into Saturn's atmosphere in 2017.
There are planetary protection considerations in systems that might harbor biology, such as the Jupiter and Saturn systems, leading to some sort of deliberate disposal of any vehicle that has not been sterilized. You want to avoid contamination by an inadvertent impact of an unsterilized vehicle on, for example, Europa or Enceladus.
There is no general procedure. If the spacecraft does not need to be disposed of, like the Voyagers or Curiosity, I expect that they will be operated until they run out of power, fail, or until they are no longer funded.
Although technically a manned spacecraft rather than a satellite, the Apollo lunar modules each carried an RTG to power scientific equipment to be left behind on the moon. However, Apollo 13's LM was brought back to Earth as a "lifeboat", including its RTG. Although the RTG was designed to be "indestructible" and expected to survive re-entry, the Atomic Energy Commission insisted that it be disposed of in the most remote location on Earth:
The intent was to leave the Apollo 13’s SNAP-27 RTG on the Moon to power surface experiments long after the crew had left. Now it was destined to re-enter the Earth’s atmosphere, along with the rest of the LM. The SNAP-27 was designed to survive re-entry without releasing its contents, but the AEC wanted to make sure it landed as far away from anyone as possible, just in case. Around eight to ten hours before Apollo 13 was due to re-enter, Lunney called Bostick up to his console and told Bostick about the AEC worries about the RTG. “I had been eyewitness to all these tests done on the RTG—it was indestructible… I reminded Glynn of that and he said ‘I know, I know, but we’ve got to put it in a safe place.’ I said, ‘Glynn, I will do the best I can do but the number one thing is getting the guys back home.’ So we did move the landing point a little bit… to put the RTG in the deepest part of the Pacific we could find,” says Bostick. Whatever was left of the Aquarius, after its descent through the Earth’s atmosphere, would find its resting place about 10 kilometers beneath the waves in the Tonga Trench. Ultimately, no released radioactivity was ever detected, despite a helicopter survey of the area.