Home fans lynched an effigy of their former idol Vermeer during the 2-1 win over Feyenoord last weekend after the 30-year-old swapped clubs in 2014.
One man was arrested and later released by the public prosecutor but is expected to stand trial over the matter.
On Thursday at a darts event in Newcastle, signs were held up by some members of the crowd with the slogan 'Kenneth must hang' while others linked him to the NSB - a Dutch political party that collaborated with the Nazis during World War II.
Vermeer is said to have filed charges against those who made threats at the darts event as well, and Ajax promised action would be taken over matters in the stadium.
"Ajax prefers a perpetrator-focused approach over the imposition of collective measures," read a statement from the club.
"The perpetrator of the much-discussed doll was arrested immediately after the match. Ajax has a total of twenty visitors in mind, and information is known for eight of them.
"It is expected that it will be very clear who the other twelve are. These twenty people are demonstrably guilty of the disorder. They can also look forward to a stadium ban.
"The length of the ban depends on the offences that have been committed."