Flat owner not paying maintenance charges

Common expenses are a charge on property and take precedence over even Government taxes such as Property Tax etc.

So a prima facie case lies. You may serve a legal notice followed by procedure for recovery in dues in Court.

As for cutting off/ reducing any essential service, it has to be for " just and sufficient " cause with a notice of 7 days. Proceedings of the recovery case should guide you on how to proceed on this.

DISCLAIMER : General information and advice provided is without any warranties as to suitability for any use, correctness and application to any specific case. Please always take proper legal counsel .  However, if it helped anyone even a little a " thanks" would be appreciated and would encourage me to keep on making efforts :-) . I am also always open to corrections and further learnings from more experienced Seniors here.

Do not cut off water supply. That can boomerang on you. You can file a civil case against her.