How to block possibility of window resizing by user? I need it for kiosk usage. First off all I want to block resizing of Firefox but another windows should not be resizing too. Is it possible?

Unfortunately I don't think you can do this, other than not running KWin.

If it's for specific applications, you can force a predefined window size in the window specific settings in the window behavior section of System Settings, then force immutability using the Kiosk settings. If you mean at a global level, as bcooksley said, it's probably not possible.