After a couple days of this annoyance a new pop-up appeared. It asked me if I would like to disable FastClick, I clicked 'Yes' (of course) and I haven't seen the pop-up since.
That was the first I'd seen this new message and the first I could do anything.
What a ridiculous advertising ploy. First of all, you can't click the original pop-up, attempting to just makes it go away. Second, you're never told what it's for or who it's by until you're asked if you'd like to disable it. By that point you're so annoyed that you're glad to get rid of it.