I dont think that during the time of the Star League a unit disregarding the Ares Convention would escape punishment.
The Star League abandoned ("suspended") the Ares Conventions in 2578 for the Reunification War, and the Star League Civil War utterly ignored the Conventions. As First Lord Cameron said, "When combating barbarians, one most fight fire with fire." (Star League Sourcebook, p. 49).
No conflict during the Star League era came close to the nearly-bloodless battles seen during the Age of War under the Conventions.
OK, during the Succession Wars, when the Ares rules were seen as "guidelines"
The Houses cast aside the Conventions in 2787, starting with Marik. Unlike the Star League's suspension, they permanently threw them aside:
"
Additionally, [Captain-General Kenyon Marik] announced that the Lyrans had abrogated the four-hundred year-old Ares Conventions, leaving him free to do so as well. Within a few months, every state had cast aside these conventions, beginning an era of unparalleled destruction." Marik Sourcebook, p.28.
The First and Second Succession Wars blew hundreds of plants off the map. The Third and Fourth Succession Wars went easy on WMDs, but they never fought according to the Conventions. Rather than fighting according to the Conventions - which would have involved very little shooting, time outs, and easy surrenders - the Third and Fourth Wars usually fought in bloody brawls.
But still: How does it play out?
They don't stop shooting until the opposition's combat units are incapable of fighting or retreat.
1) They destroy the mechs and then take the soldiers prisoner
Reasonable.
and sell them into slavery.
Slavery has never been a large part of BattleTech outside of pirates. If anything, the POWs might be ransomed back to their noble families or sent to prisoner of war camps.
3) They slaughter them all and take the equipment.
Surrendered foes tend to be spared in the Third Succession War. Slaughtering defenseless troops was a bit low by Succession War standards. "Doesn't accept surrender" means that the Second Kearny tended to keep shooting until combat units were wrecked, not that they murdered defenseless troops.