Go into the weapon guncore file as stated above and replace the scope1.dds file.You can find a blank alpha.dds here
http://forum.thegamecreators.com/?m=forum_view&t=165111&b=24
Its also called scope1.dds just replace the old one with the new one.That will get rid of the crosshair.
You might want to save the original as scope1backup.dds or somthing in case you want to revert back to having a crosshair.8-
EDIT OOPS!
Sorry. I goofed,its the crosshair.dds you want to replace, not the scope1. Scope1 is the zoomed scope, the link will get you the blank crosshair file though.