When I generate the invoice on display, I get a tax line on the header of my invoice that I no longer use and deactivate, but despite that, it keeps popping up.

Good evening, omar-olwan-gmail.

Thanks for reaching out to us here, and welcome to the Community! I'd be happy to help.

Removing the GST/HST number from invoices can be done by following these steps:

1. Head to the "Taxes" tab and select "Sales tax"
2. Click on "Manage sales tax"
3. Select "Edit tax agency setting" 
4. Completely remove the number in the GST/HST box
5. Hit "Save" then "Done"

I hope this helps! 

Feel free to reach back out if you have any other questions.