I am doing 3 years of work for client. 2017 and 2018 he wasn't registered for GST but in 2019 it is registered. I have activated GST. How to not account GST in 17 & 18?

Hi, info343.

Since GST can’t be turned off once activated, you can change your tax type to out of scope instead. You can do this temporarily to avoid GST calculations while working with your 2017 and 2018.

To change the default tax type:

1. Click the Gear icon at the top. 
2. Select Accounts and Settings then go to Advanced. 
3. Go to Accounting section and click the drop-down option for Default tax rate selection.
4. Select Out of scope of GST and click Save. 

You might want to check this out if you have further questions about GST: https://quickbooks.intuit.com/learn-support/en-us/gst-and-bas/gst-settings-overview/00/262747

I'll be around if you need anything else.