You can force sign-out a user from all Microsoft 365 sessions when you have the appropriate admin permission.
Through the Admin center in the Microsoft 365 portal, go to Users and the Active users list:
Select the user in question and, under the Account tab, choose to Sign out of all sessions for that person:
Using this option is a one-time event that will sign the person out of all Microsoft 365 sessions across all devices.
It can take up to 15 minutes for the process to complete.
The person will be able to immediately sign back in unless you have also blocked their sign-in status.
At this time, you can also reset the password or delete the user, depending on what you want to do.