And there does seem to be an equiv for those kind of wars in Traveller. That was not what the OP was about.
Really? The Zhodani-backed Ine Givar guerrillas of Efate, the plight of the chirpers, mercs hired to support or oppose local warlords or acting on behalf of megacorporations meddling in internal planetary affairs for their own profit, the revolution on Dinom - there's no equivalent? Goodness, I must have been playing a different game.
The OP was about whether there was some sort of Geneva Convention equivalent in Traveller. This has led, on the lines of evaluating the Convention and its applicability to the Traveller setting, to discussions about the background of the Geneva Convention among other subtopics. Comments to that end included McPerth's observation that the Convention was a formalization of Western rules of war and your statement that eighteenth century soldiery considered themselves, "hired enforcers for royal mafioso and thus took it less seriously then it would be in later and earlier wars," (something the Revolutionary era Americans facing British troops, and the British officers that led those troops, might not have agreed with) and that this attitude influenced nineteenth century military codes of behavior. But my observation of late 19th century conflicts that departed from those codes is off-topic?
Well, perhaps I should clarify the issue topically in a manner that can be more easily grasped. The Geneva Conventions were as much a matter of practicality as honor or humanitarianism. Whatever the attitudes of the people that advocated for them, the people that actually signed to them and agreed to implement them did so as much to obtain protection for their own nationals as to formalize notions of honorable and humanitarian conduct; they resisted measures that would interfere with their ability to prosecute wars; and they had no particular problem putting aside notions of honor when it suited their interests - which actually gave greater impetus to formalize rules in order to control such behavior.
The first Convention dealt with protections for medical services and the treatment of sick and wounded combatants. The second Convention extended those to armed forces at sea, including the shipwrecked. The third spoke to treatment of prisoners of war. The fourth spoke to protections for civilians and noncombatants.
With regard to Traveller:
There are the great powers - the Imperium, the Zhodani, the Solomani, and so forth. (Less clear if the K'Kree would be willing to accept such restrictions since they afford protections to species that the K'Kree might otherwise consider to be proper candidates for genocide.) There are the lesser powers - the Sword Worlders, Vargr states, and I don't know what else on the other side of the Imperium. And there are a host of individual systems and small polities.
There most likely exist signed conventions that serve the interests of the great powers - conventions that dictate the handling of POWs, that confer certain protections on non-combatants, and so forth. (Perhaps there are even conventions that limit the use of nuclear weapons, perhaps prohibiting their use on civilian population centers and requiring the user to conduct clean-ups afterward.) A fair number of lesser powers would most likely sign on for their own ends, though it's difficult to enforce such conventions on folk like the Vargr - likely to be more after-the-fact trials of failed Vargr commanders than before-the-fact enforcement on that front.
As to the lesser polities: while the Imperium might hold its own troops to certain standards, and while it might pursue war crimes trials against principal actors of smaller polities who offend sensibilities - acting on the "I have the power to do it, you're just gonna have to live with it" principal of justice - the question of whether any given polity actually knows of or respects such conventions is an entirely different matter. Certainly a world ruled by some extremist religion could not be counted on to honor conventions it did not sign to if it felt religious duty required other behavior.
As to mercs and megacorporations, the real question is going to be, what can they get away with?