Hello olivier-9046,
Thank you for posting in our Q&A forum.
Q: How can I kick them out and block them?
A: Would you please check if your own personal computer in the company domain? If so, you should remove your own personal computer from the company domain.
If your own personal computer is not in company domain, then if you disconnected the O365 account, then you should not see customer's logo appearing in your search bar and other changes in your windows.
Hope the information above is helpful. If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou
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