I don't believe you can remove the @ sign from this functionality since that's how the plugin is activated.
You might be able to do it by modifying the plugin code, maybe. This plugin is located under <confluence-install>/confluence/WEB-INF/atlassian-bundled-plugins and it's called confluence-mentions-plugin-1.9.12.jar or similar to this, depending on the version.
"The functionality is provided by a plugin called the 'Confluence Mentions Plugin'. If you need to remove the user mention functionality from your site, you can disable the plugin. See Disabling or Enabling a Plugin."
@Amt für Informa: I got an answer specifically to part of your comment:
I don't want to remove the mention functionality compleatly, just the @-sign.
Hi team, I’m Avinoam, a product manager on Confluence Cloud, and today I’m really excited to let the Community know that all customers can now try out the new editing experience and see some of the ...
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