Ban plastic water bottles:
Take, for instance, Pepsi's Aquafina or Coca-Cola's Dasani bottled water. Both are sold in 20 ounce sizes and can be purchased from vending machines alongside soft drinks - and at the same price. Assuming you can find a $1 machine, that works out to 5 cents an ounce. These two brands are essentially filtered tap water, bottled close to their distribution point. Most municipal water costs less than 1 cent per gallon.
So why would you pay for something that comes out of your sink.