That's all very well, but doesn't absolve you of your responsibility to new players—a responsibility every player of BM shares, but one that the longtime players should be particularly aware of.
The proper first response to a new player's character claiming to be related to a commoner is not to denigrate, insult, or argue with the character IC. It is to send a gentle explanatory message OOC giving a brief overview of the way nobility vs peasantry works in BM. (Short version: Nobles are always nobles. Commoners are always commoners. No exceptions. When an adventurer "becomes" a noble, it's because he proved that he was noble all along, just not recognized as such.)
If the player does not accept this, then there's not a lot we can do about it. However, that should be every good BattleMaster player's first response to a situation of this kind.
