Implement Dueling for PvP's sake!
Just laugh, thank them for the free kill. If you are in sPvP, harass them endlessly, force them to leave the room so REAL PvPers will join and fight. Mesmers are great for messing with the “duelers” in sPvP.
Dueling is not PvP…it is phalus waving ego boosting only. If they held dueling tournaments it would actually mean something, as it stands…it is pointless. Join the team and fight.