Bro they canTechnically if you haven't opt for OD, they can't take your balance into negative. It is their fault, they didn't charge during settlement.
They would say they can charge at any later stage if they want. It is also written in t&c which you signed. But tell them it's their policy. Rbi rule says balance can't be negative. Regulator's Rule supercedes bank's policy.
But for non maintenance charges they can't