You can activate or deactivate specific webmail clients on your Cpanel based server by browsing over to the "Tweak Settings" section in WHM (Web Host Manager).
To login to WHM, you will input this into your web browser:
xx.xx.xx.xx/whm (Where xx.xx.xx.xx is your servers ip address). You will need the root account login in order to access this.
Once you are there in Tweak Settings, scroll down to "Mail". You can select or deselect the webmail clients such as Horde, Squirrelmail, Roundcube, etc By checking or unchecking the box.