ExcludeCategories lets you exclude discussions from specific categories from the discussion view. The categories to exclude can be chosen from settings page.
You can find all sources on GitHub: https://github.com/R-J/ExcludeCategories
The addon was requested by phreak, inspired by peregrine, solution was outlined by hgtonight and the settings are stolen from Anonymouse addon. It was all here and I just had to assemble it. Thanks for that :-)