If you want to disable(or enable) seasonal skins on your server, here's a simple guide for you.

In your KF2Server.bat write next command:

?AllowSeasonalSkins=0 to disable Seasonal Skins or ?AllowSeasonalSkins=1 to enable seasonal skins when creating a server.

For example:

start .\Binaries\win64\kfserver kf-dystopia2029?-AdminPassword=123?AllowSeasonalSkins=0