What is the best way or letting someone know you're not interested?

whenever I talk to someone through email, or instant messenger to get to know someone…usually after finding one another through some online networking site…what is the best way of letting someone know you’re not interested in dating this person (assuming that is what you both were looking for)?

Just break off all contact?
Send a polite message?
Be as blunt as possible?

I am always terrible at this sort of thing. Can anyone help me out?