They only can waive AF. No bank can waive SST. Since HSBC already give you rebate/cash back with conditions, just accept it.

Yes but HSBC already offer you cash back/credit back RM25 with conditions. You still not happy? If so, you can fight with HSBC. It may track months to settle the issue.

I"m very free now, so let me share more juicy details 

Write down all the important dates and times you contact/call/email HSBC CS.
e.g.
1-Jul 11:05am called CS to request cancel. CS offers 3 swipes, but I decline to continue CC, etc.
7-Jul 2:45pm email CS to follow up, asking why still not cancel
13-Jul 1:55pm called CS again but they are not willing, etc.

You must do your work above few times with several working days gap, to show you have tried your best before approach BNM. That's what I've been doing when I burn the bridges with banks for the past many years, but that's another story for another time.

Submit these as data to BNM when you lodge your complain, providing justification I mention above earlier that SST is "prepaid" or "in-advance", then wait few days to a week or max 10 days, provided you do your homework above with complete details. Otherwise BNM may ask you submit more details. If all done in order, you will receive email from BNM to HSBC, cc you, requesting HSBC to respond within 7-14 working days 
Just sit back, relax and you will get funny replies from HSBC, so called local-global-bank-whatever 
Continue your banter with HSBC.
HSBC may still want to argue, but you stick to the "in advance".

At the end, die die to keep face, HSBC will waive it but phrase it in a way to save face and still put the blame on you 
I chose not to continue to argue with them, that it is not HSBC gesture of goodwill, but that is my right as a consumer that I am entitled to not pay the SST and to have the CC terminated. It's that simple right and we don't have to go this route. BTW, why not you try until the end and share here for other reference as well ?
Yes, I drag this SST issue until few ringgit for the late payment charges and stick to my ground and continue to play the game. In my case, I have strong CCRIS and CTOS so it doesn't affect my future credit/loan application after that.

I've shared my portion already, your call