Pretty much if you know anything about firewalls you can do this. Simply create a rule to block all outbound TCP connections to one or both of the following IP addresses: 12.129.206.130 (this is for US), 213.248.127.130 (for EU).
1.) Create firewall rule (I'll explain how to do this at the bottom of my post for "Windows Firewall").
2.) Disable it if it's enabled.
3.) Open WoW and enter account info/pw.
4.) Select the server you want to be on. Create the character if it isn't already made.
5.) Tab out to your firewall rules and enable the rule.
6.) Enter World and you are free from Real Id.
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How to create the rule for "Windows Firewall."
Sorry this guide happens to be for Windows 7. Should be similar to other Windows based Operating Systems that have "Windows Firewall."
1.) Press the Start button and open up your "Control Panel."
2.) Set your View to "Icons" on the top right corner of your window.
3.) Click "Windows Firewall."
4.) Click "Advanced settings" on the list on the left hand side of the window.
5.) On the left side of the new window, find and select "Outbound Rules."
6.) Go over to the right hand side and press "New Rule..." This is under the Actions column.
7.) Select "Custom" then click "Next."
8.) Make sure it is set on "All programs" and click "Next."
9.) Change the "Protocol type" to "TCP" and make sure the "Local port" and "Remote port" are selected as "All Ports" now click "Next."
10.) For the local IP address leave it on "Any IP address." Change the remote IP address to "These IP addresses." Select "Add" and enter the US or EU IP address I mentioned earlier and click "OK." (If you want to add both US and EU IP addresses just add the other as well.) Click "Next."
11.) For "Action" select "Block the connection" and click "Next."
12.) For "Profile" have all three check-marked and click "Next."
13.) Give the rule a name like "RealidBlizzard" or something you can remember and provide a quick Description so you don't forget. Click "Finish" and you are done!
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Also as a quick note. If you want to be seen again, get disconnected, or want to switch servers you will have to disable the firewall rule and completely restart WoW. Otherwise you won't be able to log back in because of the rule blocking you from connecting.
I use HBRlog and even with it disabled it logs into US wow every time even with Real ID Disabled(Blocked). I believe if you Switch server's though you need to Disable the fire wall block then log in, and re-block.
Hope this helps, i'll reply to any questions