Not really - unless you report it again saying "sorry, that last report was a mistake".
But, TBH, I guess this happens fairly frequently and MNHQ are used to it - after all, they've only got to read what you've reported to see whether it's a dodgy post or not, although I suppose it's a bit time consuming for them.
That's good to know. Have never intentionally reported a post so didn't know it takes you to another page where you put the reason. Seems that Maybe my keyboard-bashing is not as inaccurate as I thought, then, seeing as I haven't ever seen said type of page!