Mr Stiller has met with Ukraine’s president Volodymyr Zelenskyy as part of his role as a United Nations ambassador for the high commissioner for refugees.
The 56-year-old actor, who once in Extras played the director of a film about a man fleeing an eastern European war, said Mr Zelenskyy was his “hero”.
“What you have done, the way you have rallied your country, and for the world, it is really inspiring,” he told the president at the meeting.
Mr Penn has a long history of activism and was forced to flee Ukraine in March while he was filming a documentary about the conflict.
Russian oligarchs and those with close ties to Vladimir Putin have also been blacklisted for travel to many western countries.
The home and foreign offices have been contacted for a response.