The saddest sight of all on any weblog is a thoughtful, well written post, with a half dozen spam comments at the bottom of it. It's sad, and it doesn't need to happen.
Every worthwhile blogging software has tools for filtering spam in comments. Akismet is a very good tool if you're in the Wordpress world, and people like me who run Typepad have their Typepad Antispam service at their disposal. If your blogging software does not have a way to automatically reject spam comments, then you're simply running the wrong software.
Even with automated filtering there are bound to be clever bots (or ill-paid individuals) who sneak commercial messages into comments. As time has gone on these techniques mutate, to the point where at some gray area level you can't tell whether the comment is genuinely useful but commercial, or a real human being positive about a product.
Your comment review process should look at every single comment that comes in and be prepared to unpublish or edit it if it does not reflect your editorial judgement of what you want to publish on your site. You can only start to have an editorial process if some system flushes out all of the easy dumb spam. Turn comment filtering for spam on before your first comment goes live.