Start WoW in windowed mode.
If you are being melee attacked, jump ( it'll help to jump off of something where you will fall a small bit)
When you jump, grab the edge of your window, and resize it. (don't minimize or maximize it, but drag the edge to change the size)
The server will be tricked into thinking that your character is still falling as long as you continue to resize the window.
You'll look like you're in the same spot, but the server will think you're unavailable for melee.

hope it helpd u