A bit late to this topic, but here goes.
First off, as mentioned, word verification gets rid of most of the spam. When I first started up my blog a few years ago, I was getting hit with "I like your blog, if you want great deals on medication, click here". But, once I added the word verification, all that ended.
I would advise against moderation. Most readers when they see that won't leave a comment. And, they may not even visit your blog again. It does turn a lot of people off.
If someone leaves you a comment that seems just like they are promoting their blog, that is a different matter. And there are different ways to think about it.
One of the best ways to tell people about your blog is through comments. I've found probably 80% of the blogs I read through people commenting on my blog. And, if you see a blog you like, by commenting on theirs is a great way to get them to find yours.
If it is a blog you do not care for (is in a whole other genre, or language, or is offensive, or is just a shameless attempt to get hits to get Adsense money) those comments might be deleted.
But, by deleting comments, you can expect those people (if they are real people) to never visit your site again.
Blogs are supposed to be interactive. If not, then maybe a web site would be a better format.