Well, there seem to be apps that let you manage the notification sound (like Battery full notification), but they seem overkill for what I want.
Luckly, I found this thread in my searches: http://androidforums.com/samsung-galaxy-s2-international/331058-stopping-fully-charged-sound.html#post2819281
If you have a rooted phone, you can follow the instructions in the thread, or follow these steps in a terminal emulator on your phone:
suIt renames the notification sound, so that next time your phone wants to wake you up to unplug the charger, it will have no voice :P
mount -o remount,rw /system
mv TW_Battery_caution.ogg TW_Battery_caution.ogg.bak
mount -o remount,ro /system