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."
It's very different from how mentions work now. Basically you would have to continuously parse all the text looking for usernames. Using an @ char tells Confluence to start doing this. I know Facebook does it when you type someone's name though.
@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.
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