When writing your comment policy it is important to remember that you want to encourage comments. Define the guidelines for commenting and support a community. It also helps define what is not allowed e.g. what you consider unreasonable behavior (insults etc.) or spam. Criticizing your content should be allowed to encourage discussions, but personal attacks and insult should be a no-no.
I added my comment policy after i received an insulting comment that I did not want to publish on my site. The comment policy gave me the confidence to delete that comment, without having the need to explain myself. It doesn't only tell your readers what is allowed and what isn't, it also helps you know how to deal with the comments.
For an example comment policy I am going to refer you to my site. Feel free to use any of it on your own site :)
and now onward to the guest blogging policy.
God bless...
Shirley