Do a thorough title search on the deeds to both properties. What agreement was on the record at the time that the well was originally built? They may well have had an agreement, and it SHOULD have been recorded at the registry of deeds and made what is called a "covenant that runs with the land." So, I would not do anything until you have an answer to that question. You may be able to find a title company that could do this research for you for not big bucks.
Then, if there is a covenant, read it carefully and comply with its terms.
If there is no covenant, then you should be well within your rights to give the neighbors some sort of advance notice in writing, return receipt requested, giving them adequate time to either pay the arrearage and 1/2 the fair value of the maintenance of the well, or make other arrangements for their water supply. I'd give them 6 months.
Give them follow-up notices. If they do nothing, then check with a land use attorney. There are some quirky rules regarding water rights and land use that only a local attorney will understand and even s/he will have to double check most of them. Get a clearance from an expert in that area of the law before you do anything.