About 60 people have been turned away from the new Harry Potter play after buying tickets from unauthorised retailers, producers have said.
Staff at London's Palace Theatre have been told to refuse entry to anyone with tickets that have been resold.
One ticket for Harry Potter and the Cursed Child, which normally has a face value of up to £70, is currently for sale on a secondary website for £6,200.
Producers called the secondary ticket market "an industry-wide plague".
Sonia Friedman and Colin Callender said it was a matter "which we as producers take very seriously".
'Protecting our customers'
In a statement, they added: "Our priority is to protect all our customers and [we] are doing all we can to combat this issue. From the outset, accessible pricing has been of paramount importance to us.
"We have already been able to identify, and refuse entry to a significant number of people who purchased tickets through resale sites and will continue to track down touts and refuse entry to anyone who has knowingly bought a ticket from a tout through the secondary market.