So, whaddaya know...

Q: What if I decided to terminate the GIRO arrangement ?
A: You may write in to Comptroller of Income Tax or call IRAS Taxpayer Service Helpline at 1800-356-8300 to terminate your GIRO arrangement. You may also email to us at iitpayment@iras.gov.sg

Go figure.

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Updated 13:14 hours:

And after 2 months, "It is finished."

Finally managed to get through to someone other than a computer-generated voice and cancelled my GIRO payment... Won't get my May deduction until August though... But I'm not complaining. At least I'm getting my money.