I have experience in water law, and the legal requirements of providing water service in these situations. Please see my website: www.waterlawattorney.com. I'm in the middle of revamping it, so the "meet the attorney" link is probably the best way to find out a little more about me.

You are right - you can't take away someone's water right, which is their authorization from the state to use the water. However, it sounds like you could certainly cut off their well service by virtue of the well sharing agreement you have. They contractually agreed to pay a portion of the costs of well upkeep, maintenance, electrical costs, etc. If they are not paying those portions of the cost, they don't get to utilize the well to draw water. They still have a right to their water, they just don't have a right to use the well to get it.

Alternatively, you could simply continue to provide water, as long as someone is living in the house, but turn it off as soon as they leave and post a nice, big notice of the amount due. When new owners arrive (or the bank) requests that water service be renewed, require them to bring the account up to date first. Finally, you could file a lien on the property for the total amount due plus your costs and attorney's fees. As health and public safety are a concern when you cut off someone's water, the second option is probably the most risk adverse.

If you have additional questions, I would be happy to discuss the matter with you, or provide professional assistance, if necessary. Cell: (480) 231-4684 - Good luck!