It is definitely illegal to require a login. CAN-SPAM has many (many) faults, but it is extremely explicit about there being no funny business in the unsubscribe process.
Current FTC rules say that your unsubscribe link must either immediately unsubscribe the user or lead to a page that (at most) asks for only your email address and does not try to confuse or dissuade you.
Mark as spam in a big system (GMail, Hotmail, DCC, whatever). Seriously, it doesn't take that many pissed GMail users to cause GMail to drop all of your mail. This is one reason why outfits like MailChimp will kick you off if you get "marked as spam" too often.
(You could argue that such mail isn't "real" spam; nonetheless, "mark as spam" is realistic.)
IANAL. Unfortunately I don't think it's possible to file a civil suit on your own as with, say, junk faxes (though someone please correct me if that's wrong).
So I guess your best bet is to file a complaint with the FTC, which does occasionally sanction companies, or with your state attorney general, who also has the power to force action.
Current FTC rules say that your unsubscribe link must either immediately unsubscribe the user or lead to a page that (at most) asks for only your email address and does not try to confuse or dissuade you.
If you're fluent in legalese, it's in the set of rules known as "CAN-SPAM 2008" http://www.ftc.gov/os/2008/05/R411008frn.pdf