I recently had a bride ask me what to do when someone who did not receive a plus one, RSVPs for themselves and a guest. Eeks! This is an awkward situation but one that is somewhat prevalent (as I learned when planning my own wedding). I think that the best approach is to get back to them promptly and express regret that while you are delighted they plan to attend your budget/reception venue etc. does not allow for them to bring a guest.
For another take on this, check out this article.