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There is no "restrict". To stop someone seeing a project, remove their access to it.
Look at the "browse project" line in the project's permission scheme to see how they get access, and then make sure the user does not match any of the rules.
The easiest way to do that is with Project Roles (under Project Settings then under People). Then as Nic said, make sure the Permission Scheme for that project deals appropriately with that project role.
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