A citizen must have civic sense and has a right to use the water for which charges are paid to the authority. It may appear for him essential and for others it may appear as wastage and the observation is relative. There is no legal remedy against such mania and attitude of individuals that do not affect other's rights. Only infringement of other's rights may amount to an actionable claim.
In this case, you do not have any legal right/remedy against your neighbour unless you can.prove that due to his wasting water, you are not getting water or the water let out by them watefully is inundating your property or surrounding your property.
If above precondition to take legal action isn't there, at the most you can complain to municipal authorities.