Usage: SECURITY NONICK ADD [<nick>] PASSWORD [<password>] [<reason>]

This command will allow you to create a list of nicknames that cannot be used by anyone that is not an operator on the server or that doesn't have the password. To connect with a password you must either use

/as security nonick add *Serv password foo Services Abuse

You can use wildcards when creating your nickname mask. If you use Web* then any nickname that starts with web will be prevented from being used.