if that is really by user, then you should user Authentication.UserName.
If Authentication.UserName equals Mike, Block.
Place that just below you authentication rule sets or as bar of the authorization rules.
Through IP you can do by creating rule client.IP is in list block_IP and select action as block.under block_IP add IP you want to block.place this rule at the top I.e
Global block rule
Many thanks Michael
Many thanks R Parson