Not only is this an embarrassment for you, it is irritating for your contacts - especially those that have no desire to be on Facebook.
Following extensive searching, we have found where Facebook has hidden the page that enables you to take control and stop pestering your contacts. Sign in to your account and go to https://www.facebook.com/invite_history.php.
I have just used the 'Select All' and 'Delete Selected' options so, hopefully, this will stop! However, I don't trust Facebook to leave it there so I'll re-visit this page periodically and go through the process again, as necessary.