How can i prevent a user from logging out when a special task (like a video recorder) is running?

KDE should display a warning message that this task is still running or wait with the logout until the task is finished.
How can i accomplish this?

To my knowledge, nothing modern can be used to prevent this. There is a protocol known as XSMP (X Session Management Protocol) which should allow you to place a hold on the session to prevent the user from logging out. You would need to provide the notification dialog functionality yourself though.

Unfortunately i'm not aware of any console / script utilities that can be used to place these sorts of holds.