Please read: Contacts: View invitations

Other's have been able to delete the contact by:

Clicking Contacts in bottom left-hand corner of the page. Then, View Invitations. Once there, click on Groups followed by No, Thanks for NakedFunRoom. Does that help?

If you haven't done so already, change your Invitation Settings to something more private. As noted in Step #3 above, where it states:

"If you'd like to make broader changes to your privacy settings for all your friends, contacts, and groups, click the Change your invitation settings link at the top of the Invitations page."

As an added precaution, I would also suggest changing your password and secret question.

Hope this helps..