Difference between FlashVars & URL Rewriting?

I’m passing a variable (filename) to my flash movie so flash can proces this variable. Currently I’m using FlashVars to do this, but when I put the link to the .swf directly into the address bar and put “?filename=somethingrandom” behind it, this will cause flash to load this variable, which I don’t want.

Is there any way to use FlashVars and make it impossible to add variables to the url?