FetLife: If someone has sent you spam or tried to scam you, you can report the message right in the message window when logged into your account, or go to their profile and click on "Report" on the top right side of their page.
Lavalife: Only offers "tips for safe online dating", and strongly suggests contacting local law enforcement if you feel you've been scammed.
MySpace: For each 'unlawful' message sent to another user, the person sending MySpace will get charged $50 per email. Unlawful includes scams, as outlined by their Terms of Service. Also, MySpace has changed their terms to reflect that they no longer get involved in "Member Disputes. I quote: "You are solely responsible for your interactions with other Users, third party developers and any other parties with whom you interact through the Myspace Services and/or Linked Services so use caution when dealing with others. Myspace reserves the right, but has no obligation, to become involved in any way with these disputes." If however your specific romance scam is being investigated by the police, please send the officers to MySpace's Law Enforcement Page for more information and assistance.

OkCupid: To report a scam or suspicious email, you'll have to log into your account. Then go to the person's profile or email to you, and click on the flag that says, "Report This". Another option available is also only available when logged in: click on Contact Us at the bottom of the page --> Send Us Feedback --> I found a scammer/fake profile.