Not implemented afaik.
Easiest way would be to block processing early in client_process (i think it's there where the right click calls for merchent interaction), and issue a reject messsage instead of opening the trade window.
No way handling this with quests imho. This would allow for more customized reject messages, but requires an event to be defined (in Perl at least). Just insights, however, to good luck working on that if you intend to
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