If you really want to delete stuff in the media pool, reboot the switcher. That’s the only way that I know. Replacing any of the slots in the media pool does wipe what was in there previously, however.
The 1 M/E has 180 frames of motion it can store, no more. I’ve put pretty large TGA sequences in there so I don’t know what the exact memory size is, but probably just enough to store 180 frames of Alpha’d TGA images.