As far as I'm aware you can't disable it directly in Hedgewars yet.

However I created a little package for you in the Downloadable Content area on the game's main page.

It's in the "Hotfix" section of the page and will disable the Santa Hat permanently as well as disable the Christmas background in the menu.

Requires a Hedgewars restart after clicking/downloading.

Happy holidays!

  sheepluva <- me  my code stats -> 
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Thanks.