popcorpOk I finally figured it out after 2 hours of learning stuff and testing.
You have to remove 1 line in a ctx file which is
"TracerEffect" "bullet_shotgun_tracer01" (last line may differ depending on a weapon)
You have to find out which weapons cause red crosses, then you have to go (download GCFScape) to tf2_misc_dir > scripts > tf_weapon_<name> Extract it
Then you follow these steps to convert ctx into txt, open txt file and remove the line given above.
http://www.teamfortress.tv/post/436926/no-explosion-smoke-script
In your demo, at least the beginning of it, haven't watched the whole demo, Ambassador and Shotguns cause red crosses, so I did what I said above and removed them. http://www.mediafire.com/download/916lze989jma49a/txtfiles.rar
Put those files into \tf\custom\my_custom_stuff\scripts It all worked for me, hope it works for you too, if something happens, feel free to contact me.
holy shit, it actually worked. thank you so much man.
edit: some demo files still have some red crosses but hopefully i'll be able to fix it if i follow your instructions.