In my world, the government has prohibited owning your own military-like force. So what is a filthy rich person supposed to do to get back at the one who has given them a deadly insult? Hire mercenaries!
There is a well-established and respected mercenary guild, with few (if any) mercenaries operating outside of the guild. If you hire mercenaries from them, you'll be guaranteed professionals who won't give you anything less than their best. If your adversary also hires from the mercenary guild the mercenaries will use non-lethal tactics against each other, but with the reputation of the guild at stake you are still guaranteed that the mercenaries won't give you less than their best.
What regulations from the government and/or self-regulations from the guild can prevent this system from being abused? I want to avoid situations where a richer person could force a less wealthy rival into a fight in order to have the rival spend money they can't afford to (or kill them off if that's allowed as an outcome). However, the system also needs to be able to provide satisfaction - leaving a deadly insult unpunished could lead the insultee to attempt more drastic measures. In any case, the mercenary guild does want to ensure its own profits, but it definitely doesn't want the government to feel the need to step in.