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.
Two vulnerabilities have been published for Confluence Server and Data Center recently: March 20, 2019 CVE-2019-3395 / CVE-2019-3396 April 17, 2019 CVE-2019-3398 The goal of this article is...
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