I have tried this from this article but service desk team members and admins can still see the projects in portal https://community.atlassian.com/t5/Jira-Service-Management/Turn-off-Service-Desk-Portal-per-Project/qaq-p/682015
You can go to "People" and check if these people are registered there as "Service Desk Team" or "Service Desk Customer".
If they are, remove them and validate.
Some things through project role actions users can continue to have visibility in the portal.
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