As for the OP, I'd honestly think that 'no genocide' would be implied, and that you wouldn't have to actually have an official rule for it.
You would think, but the simple truth is it would be incorporated into the official laws - it's not a rule if no one ever states it's a rule. The US, for example, passed the Genocide Implementation Act of 1987 in late 1988.
But, General Order 24 allows for complete destruction of the surface of an inhabited planet.
And there are several situations that might allow for it legally speaking:
SR3P12 (Starfleet Regulation 3 Paragraph 12) - In the event of imminent destruction, a Starfleet Captain is authorized to preserve the lives of his crew by any justifiable means. This is quoted in the Voyager episode "Equinox"
So have a gander...
Admiral Jisil T'ror
“Does anyone remember when we used to be explorers...”