Hi,

I need to allow only one browser on the system (Firefox) and I need to remove/block all other www clients: browser widget & other browsers. Other browsers are not the problem, but the widgets. If I would remove default browser widget from KDE, user could still install new one. How can I block it?

I guess what is the intent of removing all other browsers and browser widgets? Are you setting up some kind of Kiosk where users can use Firefox and that's it? If this is the case, you might take a look at KDE's Kiosk functionality