How to deal with Internet bullies?

creyes123 writes: "I run a free website with an online model airplane design calculator. The number of registered users has quickly climbed and I've gotten many compliments. Out of nowhere, a fellow shows up and proceeds to bad mouth the calculator in a posting in one of my forums. After I politely point out that he's mistaken and should have looked at the documentation before posting, he changes the subject and bad mouths a different 'flaw'. The cycle repeats a few more times, with no apparent end in sight. I want to encourage folks to share their opinions, but constructive criticism was clearly not his goal. I feel that the whole episode was just a massive time waster for me. What did I do to deserve this? Could I have handled this better? Help!"