Misunderstandable error text for posting answers

I configured the plug-in in the way that one user can answer once per question, but after posting answer I can still see the box which provides me to type the answer! If I do it again, I will see the error “Your answer must have 5 letters or more.”, but my second answer contains more than 5 letters, so the best solution to hide the box if one answer is allowed per question.
Thanks!