Hi @quentin, welcome to the Atlassian Community.
You could restrict access to any project and add the user to one of the Insight roles only. This way the account would not see any project.
You'll have to make sure no projects use the "any logged in user" role in the permission schemes and that the account does not belong to any group that's being used in a permission scheme or project role directly.
Depending on you current setup this could need a lot of rework in your permission schemes.
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