Hello,
1) If you have passed a rule following appropriate procedures and if the notice of the same had been given to all members the decision is binding on them.
2) You can issue a legal notice on the defaulting members demanding payment of the arrears with interest or face appropriate penalties.
3) As you are not a registered housing society you cannot move recovery proceedings against the defaulters before the Registrar of Societies. After passing a resolution in the Society's general body meeting you can file a civil recovery suit as your society is not registered.