This may help sometimes:

Open your package manager (Synaptic?), search for packages including
"compiz" in their names and removes them all (support for 3d desktop
that's may cause difficulties, personally i am better off without them).

If it's doesn't do anything, you can still install them back.


Otherwise you may try to see the tiny documentation of gnome-screensaver
by typing man gnome-screensaver in a console, I wonder if the command:

gnome-screensaver --no-daemon

would stop the screensaver from showing up....