Human Spam Comments - Where to Draw the Line?

The arms race between spammers and bloggers continues and I often feel like I'm on the front lines.  I'm so thankful for tools like Akismet that automatically prevent bots from spewing bullshit in blog comments.  

However, there is an insidious human element that has been rearing it's head with greater frequency. These are commenters that appear to be leaving an on-topic comment, yet use a link and name that reflects their true motive - to make a web site appear higher in search results. Many of these comes from spam farms where people are paid to post honest-looking spam comments. [See Jake's weird experience with a spammer.]

Here's an example from this site:

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This kind of comment presents a sticky problem.  The commenter understands our motives and gives us a nice compliment.  Yet, in my view, they are much more motived by the "Phone Cards" link than communicating with me. I deemed this spam, reported it as such and deleted the comment. Another example:

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Again, they spent time to understand the content and say nice things.  Yet, again, I have to assume that the only reason they left the comment was for the "page rank" link.  Reported and deleted.

The hard part is when well-meaning commenters are using tactics that are reminiscent of spam.   Example:

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In this case, the comment was very much on topic and it was (mostly) obvious to me that the priority was communicating with me vs. dropping a link. I left this one alone, even though the link to "Computer Consultants Kit" is quite suspicious.  

I've been struggling with this for many months.  My instinct says to have a zero tolerance policy and delete anything that resembles spam. Yet, where do you draw the line?

For instance, if I leave a short comment on someone's blog and use the name "Common Craft", could I be guilty of spamming under my own policy?   Does it matter that it's linked to my company vs. a 3rd party? What if the link was "Plain English Videos?"

Right now my policy is to have a hair trigger when it comes to comment spam.  If I even catch a whiff of the stuff, the comment is deleted.  I refuse to become a means for low lifes to get a free ride. I'll stop now before I get too emotional.

Do you have a policy? Where do you draw the line?