Hello @Sidi Mohamed ABED RABOU , Thank you for reaching out to us.
I have few suggestion and questions, please help me in understanding:
1.Have you logged in with Global admin to the portal and trying to delete?
2. Is Delete option is greyed out for you? If not please help what is the error you are getting when you click on delete
3.You may try the following steps mentioned in the article to delete the user from your org:
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/leave-the-organization
Please upvote this answer if it help you or feel free to reply with your questions if you have any doubt or i misunderstood the ask.