> The closed systems were better able to fight spam because they could easily ban people and IPs.
Unless the server allowed one to send email or post to usenet without having to log in first, then there's no reason why the provider couldn't simply disable the account or block the originating IP from connecting to the server. From what I can tell, the providers weren't interested in blocking spam by blocking IPs or disabling accounts. This is very similar to the robocall problem and phone companies not really trying to fix it.
Unless the server allowed one to send email or post to usenet without having to log in first, then there's no reason why the provider couldn't simply disable the account or block the originating IP from connecting to the server. From what I can tell, the providers weren't interested in blocking spam by blocking IPs or disabling accounts. This is very similar to the robocall problem and phone companies not really trying to fix it.