yes you can, each app has its own option to handle this
in the case of smplayer/options/preferences/general/video tab/disable screensaver
enable that checkbox
each app handles this differently so can't give you a gide that will work for all apps
in kaffeine, i can't find the option in kde4
the screensaver usually should not be enabled while watching videos on youtube but since the last time i used a screensaver was in 1998 can't say anything about it
do you have reason to have a screensaver enabled? security perhaps?