No policy really but as only members of my community can post blog comments that cuts down a lot of spam. The odd spammer registers and starts a full-on spam blog though, but that another matter all together.
In the past couple of hours I've had 2 attempts at commercial ads via my comments. One of them was from a company that obviously competes in the same space as my main source of income (and no that isn't adsense!)
The other one is a lot more worrying and if I allow it to be published could lead to legal battles etc.,