I am absolutely zero when it comes to technical topics, but I can hardly imagine that it be impossible for servers to exclude single computers - regardless of the User Name apllied.
Consider this an education. We cannot idenfity individual computers. Computers just don't send out their processor's unique serial number with every HTTP request, or anything like that... and if they did, they might just be faking it. Any form of ID 'cookies' and the like can be erased. The IP address is really the only thing that can't be faked. We can only identify individual IP addresses. On the Internet, many computers may share an IP address (ever hear of 'proxies' and 'NAT', 'Network Address Translation'? Look them up on Wikipedia) and many computers can jump around between many IP addresses (dial-up networks and AOL users, in particular). The end result is that we cannot block them by individual computer: we can only block them by their IP address, and (if the IP address is variable) by their internet service provider, and there are a *lot* of people out there who can be affected by such a block, particularly in the case of AOL.