Introduction
If you want to restrict cPanel users from using Roundcube or Horde, you can disable said Webmail client altogether. Users will still be able to use the other Webmail client.
Procedure
Log in to WHM as the root user or a reseller with root privileges. Go to Tweak Settings and find "Enable Roundcube webmail" or "Enable Horde webmail" and set it to 'Off'.