If you wish to cancel the card, they will just waive the GST, since you never activate and use the card.

the CS cannot reverse the GST charge. but she can help you.
very simple, what she need to do is, let u sign a form since you already went to the branch.

for sure the bank can waive the GST, provided you never use the card & if you really do not want to keep that card.

(but however now many banks give rebate or other form of compensation for the GST)