how to remove office 365 trial before activating

In your example above, it does not matter that 'UserA' has activated a copy of Office on a PC being used by 'UserB'.

One of the major benefits of subscription-licensed Office is you are pretty much always compliant with the licensing, as long as you have Office 365 ProPlus subscriptions for all of your users. I must point out that what you are trying to do seems like a pretty pointless exercise, unless the first User has hit their 5 device limit and needs to deactivate an old PC move on to a new one. So they can take care of deactivating old installs on their own without admin intervention.

Regular Office 365 users can self-manage their ProPlus installs by signing into the Office 365 portal (). This is a relatively new feature, so you may find a number of outdated blog posts and documentation that says users must deactivate their installs individually. Office 365 admins can list the activated Office 365 ProPlus installs for a user and deactivate them via the user accounts list in the admin portal. Once the 'subscription expired' message appears, the new user can enter their Office 365 credentials to activate the install in their name. If the PC has no Internet connection when it is deactivated). However, there might be some lag for the client to detect the deactivation (e.g. In my testing, the client shows a 'subscription expired' pop-up the next time Office is opened. Once that happens, the installed copy of Office will deactivate on the client.

You need to deactivate the Office instance in the Office 365 Portal. It's Microsoft's servers which are the canonical source of the subscription/activation info. Even if you reinstalled Office, that does not remove the computer from the original users's account. Subscription-licensed copies of Office are associated with the user's Office 365 account.

How to remove office 365 trial before activating