Hi me,

Here is the service that manages it and here is the default contribution.

You just have to override the default contribution for the searchbox group suggester and remove the user and group suggestion :

<component name="org.nuxeo.platform.customer.userandgroup.suggester.disabled">
  <require>org.nuxeo.ecm.platform.suggestbox.core.defaultSuggesters</require>
  <extension
    target="org.nuxeo.ecm.platform.suggestbox.service.SuggestionService"
    point="suggesterGroups">
    <suggesterGroup name="searchbox">
      <suggesters>
        <suggesterName remove="true">searchByUsersAndGroups</suggesterName>
      </suggesters>
    </suggesterGroup>
  </extension>
</component>