To cancel your Australian Business Number (ABN), you should first ensure that you've met all lodgement, reporting, and payment obligations with any government agencies you deal with.
Once you have fulfilled these obligations, you can cancel your ABN online using myGovID and Relationship Authorisation Manager (RAM). Here is the link to get your ABN cancelled - https://www.abr.gov.au/business-super-funds-charities/updating-or-cancelling-your-abn/cancel-your-abn
The process is immediate, and it will also cancel any registrations for goods and services tax (GST), luxury car tax (LCT), wine equalisation tax (WET), and fuel tax credits (FTC) associated with your ABN. If you're unable to cancel your ABN online, you can contact the Australian Business Register or lodge a form by mail.
Remember, if you want to cancel your pay as you go withholding, you must do so before cancelling your ABN.