Hi there. Can I deactivate @mentions in Confluence? If yes, can any user do that or only an administrator? Thanks! 
Yes, you can. In the following information you can see how. At the end of the first document, under “Notes” it is explained that you can deactivate the plugin.
https://confluence.atlassian.com/conf54/confluence-user-s-guide/sharing-content/using-mentions
https://confluence.atlassian.com/display/UPM/Disabling+and+enabling+add-ons