Flush it out
Two things will prevent the smell bleach and using fresh water. If they have access to a hose they may try to open the valve and run fresh water into the tank from the vent next to the valve. The male end of the hose just fits inside the opening. Not sure how long it would need to run to change out the water 100%, a while I reckon. Then add bleach I use about two cups each time on mine, I am afraid of what it may do to things I cannot see inside the ballast tank. If that does not work pull the boat out, open the valve flush out the dirty water with fresh water the launch and fill with fresh water again. The water out here in Mississippi is the most dirty I have ever sailed in and mine still smells fresh.
