Is there a way to disable the use of ligatures? Some of them look just wrong, especially when spaces get compressed to a mere nothing and I would rather not have them at all.
The only way I've found so far is to choose a font without ligature support, but those seem to be rare among the better looking fonts.

Whilst it appears that the KDE configuration interface doesn't support this, it could be possible that the underlying system (fontconfig) which KDE shares with other X applications may support this. It stores it's configuration at ~/.fonts.conf

Most editors offer the option to insert ligature breaks during composition which fontconfig appears to recognise; presumably the authors of the texts you find problematic omitted to do this.

The problem with excluding ligatures generally is that it means you cannot then use them when it would be desirable.