A vulgarity-like system wouldn't work. How would it be reported? *What* would you report, and how would you convey all the background behind it? You'd end up with the challenger saying he had a good reason, and if the challenged gets to respond as well, they would say the challenger's reason is bad.
The way I had imagined it that the reason box, that you have to fill in as a challenger, would be sent to the deciders. So the emphasis is upon the challenger to present his case in that box. If the person refuses the challenge, the challenger has the option to choose to allow peers to decide on whether it was right/wrong for the defendant to refuse such challenge.
Lets say that three random nobles form across the continent are chosen. If your example was in the reason box, they could say yes, this is a valid reason, or no, thats a stupid reason.
If it was a well thought out, RP'd reason (Im sure most spammers would not put much time into it), they could say this is a valid reason, the decision is then told to the two participants, with the option to have a duel. If the defendant refuses again, then he is publicly shamed (in line with Toms rule of leave it to the players/social to deal with). Whether this is just in the realm, or continent wide, I dont know.
But it would not result in any loss of honour/prestige but it would provide RP ammo against that person.
