Aloha Mal, I'm Greg, an installation specialist, 10 years Windows MVP, and Volunteer Moderator here to help you.
What is the make/model PC and how did you turn off touchpad before?
Do you want it permanently turned off or just when you connect a USB pointing device/mouse?
To permanently turn it of go into Device Manager > Mouse > Touchpad Device > Driver tab > Disable. Every device has Disable. Be aware that you'll need a mouse (or touchscreen) to get back to that setting at any time.
To turn it off from PC hotkey or when USB mouse is connected requires having the latest software which the Update might have overridden or replaced. Update the touchpad driver from the PC maker's Support Downloads webpage. Then go into Windows Settings > Devices > Touchpad to go over all settings, including Additional Settings where the software will usually add a tab that has more options including USB connection options.
Also, If this was caused by Windows Update(s) then you can check which were installed at Settings>Update & Security>Windows Update under Update History, then uninstall them from the link there, and immediately check for these again and hide with the Hide Updates tool explained here: https://www.howtogeek.com/223864/how-to-uninsta...
You can also System Restore to a point before the Update following this guide: https://www.windowscentral.com/how-use-system-r...
If this doesn't work you can use the new Uninstall Updates button in Repair Mode using Option Four here:
https://www.tenforums.com/tutorials/5486-uninst...
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I resolved your problem, I will keep working with you until it's resolved.
________________________________________________________
Standard Disclaimer: There are links to non-Microsoft websites. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the sites before you decide to download and install it.