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."
On average, around one person per day has been turned away since previews of the play began in early June.
Tickets for the two-part show have been in huge demand, and fans have cursed touts who sell them on for inflated prices.
Re-selling tickets is not illegal, but the producers warn that any tickets that are advertised for sale on the internet, in newspapers or elsewhere will be "automatically void".
Only those bought from the two official ticketing platforms - Nimax and ATG - are allowed.
The show is sold out, although there is an online lottery every Friday in which 40 tickets are released for every performance the following week.
At the time of writing, one ticket to watch one part of the show from the stalls is on the Viagogo website for £6,213.76, while Stubhub is offering two tickets for both parts for £4,999 each.
Viagogo said prices for popular events can be higher "because there is huge demand and limited supply".
"However, while a seller can list a ticket at any price he likes, it doesn't mean the ticket will actually sell at that price.
"Tickets listed at silly prices rarely sell. The reality is that around half the tickets sold on Viagogo are priced at or below face value."
StubHub has previously said its sales were "driven by supply and demand" and that sellers "set the price for what they think the market will bear".
Meanwhile, the script of the play, written by Jack Thorne and approved by Potter author JK Rowling, has become the UK's fastest-selling book this decade.