It shouldn't be allowing them to search for profiles if you have that setting turned off in the Global permissions page of the Confluence admin area.
If you still keep having issues then this plugin may help:
In Confluence, through the Global Permissions (Confluence Admin > Global Permissions), we have the option to uncheck the View User Profile box in order to not allow the anonymous to see it. However, if you are talking about the usernames to be still searchable after unmarking that, I would suggest you to vote and also share your thoughts through this improvement request, since we are working to implement it as a bundled feature.
Hope it helps you!
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