if you decide on a sign, be sure to use small words and BIG lettering, just in case they're drunk enough to have a hard time reading.
and tell the son that you have a contract for him to sign, which states that he's responsible for any damages to the property and tub when he decides to have friends use the tub...then ask him to sign it...in blood.
my son would FREAK and think I was 100% serious.
have you considered having him call these friends and have them do a SUPERVISED drain, clean and refill so they know the work involved when they want to be schmucks?