As well as what Aron said, you need to REMOVE "browse projects" from any other groups those users are in, or any other way that they might be getting access to them.
Generally, to do "only see project X", you need to re-engineer the default groups and permissions in Jira, because it's permissive and simply doesn't do these sort of restrictions. You have to tear it down and rebuild it from a restrictive angle.
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