An allegation of online abuse was made following the match on November 18, leading to an investigation by the club which was eventually closed after the player in question, El-Hadji Diouf, said that he hadn't heard any abuse.
However, Sky Sports News showed footage of the Senegalese striker being abused, leading the Championship club to reopen their investigation.
"Like all Millwall fans and football fans generally, we're appalled by the footage we've seen. Clearly that behaviour is unacceptable in any football ground in the country and at Millwall we will ban the individuals for life and we hope that Sky will pass the footage to the police so that further action can be taken," said Millwall chief executive Andy Ambler.
"Sadly racism exists in our society and certain individuals want to bring it into football grounds. It's down to us in football do to more. We think football is the solution to the problem and the work that we do will continue and we need to continue it. That day against Leeds obviously there was an allegation of abuse made online initially. We investigated it straight away.
"We interviewed stewards, police, players, we looked at the TV footage too as it was live on Sky. After speaking to the player who had the alleged abuse at him during the day, he said that he heard no abuse on the day so that was basically at that point the end of the investigation - although we're going to open it again now."
The game ended 1-0 to Millwall.