How can I get rid of this spammer?

A couple of days ago I began seeing an Empathy notification as soon as I log in (Ubuntu 12.10 64-bit) which disappears after a few seconds to be followed by a Subscription Request dialog box with the same message plus 2 option buttons:

<email address hidden> would like permission to see when you are online
Decline Accept

I cannot get rid of this nuisance even though I've pressed the Decline button many times since it began.

As I have only Google and Facebook as my online accounts, this is probably coming from Facebook chat. But I cannot see any trace of it when I login to my Facebook in my web browser.

Can someone please advice me what to do?

A combined screenshot can be seen here: http://i.minus.com/ibLbynalmL3Ne.png