Once GST is turned on, it can't be turned off. However, if you recently set your QuickBooks Online file you can reset your file and start again (provided it us under 90 days old).
If your file is older than 90 days or you do not wish to reset your file, you can change your default tax rate to Out of Scope and your transactions will be posted to GST.
For assistance on changing your default tax rate, see How to I set up a default tax rate for transactions (Inclusive/Exclusive of GST)?