The character, played by Gareth David-Lloyd, was killed off in the fourth episode of Torchwood mini-series, Children of Earth.
Moffat wrote on Twitter: "Ianto fans only: Russell [T. Davies]'s character, and I thought his death scene was brilliant. Not reversing it. Stop asking. [sic]"
In response to a further fan question, the writer added: "Torchwood is Russell's - nothing to do with me."
Moffat took over showrunning duties on Doctor Who from Davies for the show's fifth series, which aired earlier this year. However, Davies still serves as an executive producer on both Torchwood and second spinoff The Sarah Jane Adventures.
Doctor Who returns to BBC One this Christmas. A fourth series of Torchwood is expected to begin filming early next year.