How to prevent Office 365 overiding Office 2019?

I am the IT administrator for a group with several computers, each used by multiple users every day. We have individual perpetual licenses of Office 2019 installed on each PC's. The computers are not hooked up to the internet for long periods of time. It is not practical for us to use Microsoft 365. We uninstalled Microsoft 365 before installing 2019, so that is the only version we have. We want to use Office 2019 at least until the next release of the perpetual version of Office after Office 2024. We are choosing to go without support until we upgrade.

We have a few users who are taking college classes and have a Microsoft 365 email/account for school. Sometimes, not every time, when logging into Microsoft 365 in a web browser not the app or Outlook, it causes Office 2019 to switch over to Microsoft 365. Sometimes it does this on its own. I assume this should not cause it, but it does. Most times, we need to uninstall both versions of Office and reinstall Office 2019. Sometimes we get lucky and are able to resolve the issue of having Office 365 with a simpler solution. The number of times we need to do this has been increasing. The time to try different solutions until one works has been increasing as well.

I have looked at other questions from people in similar situations, and the answers deal with how to fix this when it happens. Is there a way to prevent this from happening? Is there a setting in the Group Policy Editor or somewhere else to stop this?