The 26-year-old actor was visiting the Hobgoblin venue in New Cross, South London, with his "Nymphomaniac" co-star Mia Goth when the incident took place.
He posed for pictures with several fans, but things turned nasty when someone grabbed his hat and refused to give it back, reported Sun online.
The pair was then involved in a scuffle.
"Her (Goth's) mum lives just down the road. She came down to see her mum and he tagged along. He did get a bit frustrated and there was a bit of a scuffle," landlord Mark Harris said.