Well it has been an extremely long time since I used 18.8 on my old XP system and it didn't give a graceful exit, but as far as I can recall it did have the X top right (and the key stroke option I suggested did work. Just in case, there are two Alt keys on most keyboards, it is the one on the Left side of the keyboard.
However, it would appear something has changed, which it shouldn't have in what is effectively a closed version.
Again previously you could ignore the nags and after 13 days (I can't recall the exact time frame), but eventually it would renew itself.