As user issue rm -rf ~/.kde4/share/apps/nepomuk or use dolphin to remove that folder and get rid of all nepomuk data.
If you want to play nice you can disable nepomuk before doing so and re-enable it afterwards.
For some distros it might be ~/.kde/…
And BTW, please do not confuse nepomuk with akonadi which did not have years yet to stabilise and did in fact get quite stable within a short time and works very well in 4.8 – unlike nepomuk.
Forgot the most important part:
As user issue rm -rf ~/.kde4/ share/apps/ nepomuk or use dolphin to remove that folder and get rid of all nepomuk data.
If you want to play nice you can disable nepomuk before doing so and re-enable it afterwards.
For some distros it might be ~/.kde/…
And BTW, please do not confuse nepomuk with akonadi which did not have years yet to stabilise and did in fact get quite stable within a short time and works very well in 4.8 – unlike nepomuk.