Instead of having sponges for water donaters only, how about mayor+. It would be a great perk to being a mayor, and what does a res/builder need a sponge for anyway. Sponges are mainly used for big underwater builds, and few residents and not many builders will get the oppertunity to have big underwater builds without first haing a town. Please take this into consideration. This is an easy way to stop the grief.

Or maybe even have both. Either wait until you are amayor, or buy the water feature. That way only dedicated or at least people who are dedicated enough to donate will be able to use sponges in peace and harmony.

 

Now that I come back a day later, this is a terrible idea.