Would it be possible to add an extra field to the commenting system that requires users to add a 3, 4 or 5 figure number? The length of the number could be selected randomly and all you would need to do is add 111 or 2222, any number would do depending on the length of number the system asks for. This should not be too easy for spam bots to mimic - especially if it were to be requested after someone has written a comment. Text readers should be able to deal with this too so site accessability would not be affected.
PS WordPress.com is a great blogging platform!