Freehold outlet the Black Bull Inn in Doncaster, South Yorkshire, faced the problem last week when a couple threatened to leave negative TripAdvisor reviews unless they received a discount.
They had dined from the two or three-course Sunday lunch menu (£10.95/£13.95) without complaint but were unhappy when billed for two courses having had only one.
After a debate £2 was taken off the bill and the couple left, but two one-star reviews appeared on TripAdvisor later, slamming "appalling customer service" and "terrible management".
"They said they had enjoyed their meal but refused to leave without a discount," said Dawn Morely, partner of the owner. "They did pay but we had to knock money off to get them out. We have contacted TripAdvisor but we don’t know if they will be removed."
The Good Food Guide consultant editor Elizabeth Carter said: "This is a problem with TripAdvisor. People are learning how to use it to their advantage — it is blackmail. I think it is a trend.
"There is no easy way to deal with it other than expose it and create negative publicity for TripAdvisor. Establishments should stop taking it so seriously and play up other sources in its place until it is moderated."
TripAdvisor UK Communications director Emma Shaw said: "We take allegations of blackmail or threatening behaviour by users against restaurants very seriously. If an owner experiences this we urge them to contact us immediately."