How do I disable/enable WHM access from the command line ?
If you wish to disable access to WHM for all users, block incoming traffic on port 2086 and 2087 (this will lock you out as well).
You can also use a firewall to allow access to those ports for specific IP addresses only.
You cannot turn an account into a reseller account (grant WHM access) from the command line. However, you can use the XML API to automate this process. See http://www.cpanel.net/plugins/devel/ for more information.
You cannot shut down WHM, as the WHM services are used to ensure the server stays functioning.