Co-owner Sullivan's controversial decision to describe the performance against Wolves as "shambolic" angered Zola, and defender Matthew Upson said the comments would not help.
The letter was seen by some as a move designed to inspire the players to prove Sullivan wrong.
Zola now wants to forget the row and insists he is concentrating on on-the-field matters only.
"Whether it helped or not I don't know but for me it is the end of the story," said Zola.
"My mind is on other things. If I go back to those episodes I can give you my opinion but it's not necessary.
"What has been is not important. The focus has to be on Sunday's game."