Can an unregistered association/society force to a flat owner to pay maintenance charges?

Yes. You shall raise this among other flat owners and RWA shall be registered under the act of WB.  You shall object to the management how they can collect funds without forming a society and how are they call as Management?

To my knowledge, there is no legal requirement that every society or association is to be registered. However, the onus rests with the GB of the Association that it is duly registered.

A registered resident welfare association alone can be able to enforce the bylaws of the cooperative society meant for flats or apartments.

An unregistered association cannot force the members to pay maintenance neither then can sue the defaulters.

You gather like minded people and strongly object to the demand and refuse to pay any