This is much simpler than many would lead you to believe and can be controlled well with built in WP functions.
Doing some reverse social engineering on what Spammers desire, it is almost always to post a link to some badsite.com on your blog....
So... go to the admin panel>settings>Discussion then look for 'Comment Blacklist'. In this box add lines of text similar to:
...Add what you want to block...
The result will be that anyone (or anything) trying to post a URL on your site will have their comment marked as comment spam. (Clean these out a regular basis, at your convenience)
I combine this with setting 'Comment Moderation' to 1 (any comment that has one URL or more is held for moderation and set 'Before a comment appears' to 'An administrator must always approve the comment '.
I then use the excellent App WordPress for Android (Other versions available) to quickly review comments that come through (that are not spam) for approval.
I also set comments to close out after a specific period (extremely useful on blogs with many comments) and 'comment author must fill out name and email'.
Last, delegating someone with a smart phone, the app, and an understanding of what will be allowed as a comment on your site is fairly cheap and efficient. The spam comments do not even show up in the app making it much easier to review them!