How much cash allowed to loan prepayment to bank

Section 269ST meaning you should make payments below 2 lakhs or use non-cash modes for amounts of 2 lakhs or more. So you can make the full Rs.15 lakhs cash payment for prepayment/foreclosure without breaching Section 269SS/269ST, as the exemption applies to banks. 

Legally, cash payment of ₹15 lakh to Punjab National Bank (a banking company) does NOT violate section 269ST, because banks are expressly exempt under the proviso to section 269ST.
Section 269SS is also not applicable, since you are repaying a loan, not accepting one.

Practically, however, PNB is very unlikely to accept ₹15 lakh in cash due to RBI AML/KYC norms and internal bank policies, and may insist on account-payee transfer instead.