Hey there, @000
There isn't a tool to provide that, but that would be a good feature request.
You could always use Host Access Control tools to limit the IP addresses that can access various resources on the machine, but this would affect every domain on the system, which may not be what you want.
The easiest way to accomplish this would be to just change the user's password in WHM >> List Accounts so they can no longer access cPanel. They would still be able to use email and make a direct connection to webmail.