While you're still having internal debates on how to handle spammers, perhaps you could add the link http://subscribe.wordpress.com to your error/success message (the message that users/blog owners have no control over)? Despite my test addresses possibly falling into the single use bucket, I still received Success messages when I re-submitted the address through the form.
It seems like quite a service gap to ask someone to hold on to an old confirmation message - maybe the first time it went to their junk folder. I mean, the first confirmation message may have been sent a year ago! Perhaps you just need to have a timeout setting to deal with spammers? One message per day to potential subscribers? Your obviously keeping a record of all attempts so adding a timeout wouldn't be too much extra work/extra data.
And just an additional note to further illustrate the situation - I have tried the subscribe.wordpress.com link above and entered in one of my test addresses. Lo and behold, there was a pending subscription notice from the 10th of January. I never received that email! It is not in my Deleted folder, which contains messages older than the 10th, and it is not in my Inbox. So, what is a user to do? I re-submitted that address multiple times, and due to your policy of course, never received an additional message. From the Subscribe page, I deleted the Pending activation and then re-submitted the email address and this time I received a confirmation email! That's a lot of work to put a potentially unsavvy web user through. So how do you account for all of the potential gaps in the email chain? Again, I suggest that you should alter your anti-spam policy in some meaningful way perhaps using a timeout feature to make addresses valid again. And again, I suggest an interim solution of adding a small note about the Subscribe URL in your Success/Error messages which are built in to the function.
I will use your private support form to give you the viewing password to my site so that you may look at the subscribe feature which is on the front page.
Thanks for your help, I appreciate that you have at least communicated with myself (and angelagoodnight).