Very similar to "Q: How to disable "Change domain name" functionality?" in https://forum.directadmin.com/showthread.php?t=58062. Command name: CMD_FILE_MANAGER. Regarding skin itself - it can be done using "Customize Evolution Skin", if you just want to "hide" the URL, but that'd still let your customers call CMD_FILE_MANAGER from URL, so, never_commands list is the best option to prevent calling CMD_FILE_MANAGER at all.