Citizenship can be withdrawn.
He could be banned on the grounds that he was a KAPO criminal and collaborator, aiding the Nazis against his own Jewish brethren during WW2.
Even though he admitted to being a KAPO and enjoying it, just a trial or even a public commission’s hearings/report on his activities might discourage all but the most Left organizations from associating with him (though he would send them money through various fronts, of which he is very good at doing).
He’s too old to give a damn about the exposing of his past, but it could do a lot of damage to his legacy image and perhaps cripple what his leftist son is continuing to do.
Otherwise, just try him for war crimes and Nazi collaboration and sentence him to death, in absentia. That would give the Hungarian government grounds to confiscate all his money, operations, holdings, etc. for distribution to those Nazi victims he betrayed.