Any process in which a virus stops, prevents, or reduces the activity of host IRF9 (interferon regulatory factor-9), a transcription factor involved in the innate immune response. Viral infection triggers binding of IRF9 to phosphorylated STAT1 and STAT2, forming the ISGF3 complex. The ISGF3 complex migrates to the nucleus and activates transcription of IFN-responsive genes.