Tor is an anonymous web browser, as I understand it. It does not send information like HTTP headers which are required to identify things like the type of browser you use, the capabilities of the browser, or what page referred you.
All web browsers send these headers. Spammers fail to send these headers because of bad programming. Without them you look like a spammer.
If you choose to use Tor, you have to realize that it restricts your browsing freedom.
Use a normal browser, log in and ask the sysop to loosen the restrictions on his site. The plugin can disable these kind of checks. Tell him that he will get lots more spam, but that you will be able to make comments using an anonymous browser.