how do I limit access to a single a project?

[Elizabeth Karam]

This question is in reference to Atlassian Documentation: Managing project permissions

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[Nic Brough -Adaptavist-]

If you mean you want to remove a user's access to a single project, you need to remove whatever permission it is that is currently letting them in.

Read the permission scheme for the project and look at how it is granting access (browse project is the most important line).  Then make sure the user no longer matches that rule.