You can request the closure of the remaining instalments at any time, but it is important to know that you cannot request only partial closure of certain instalments of a transaction (for example, if you have made a payment in 6 instalments and you still have 4 to pay, you cannot close only 2 or 3, but all 4 outstanding instalments are closed). 

After the instalments are closed, this amount generates an obligation to pay the following month, but the amount can be paid even sooner, even before the instalments are closed early. 😉   

You can request closure either through BT Pay or in a BT unit.