1) Open an elevated command prompt (Start, type in cmd, right click, open as administrator)
2) In that elevated command prompt, type assoc .ext= " and press Enter. (there has to be a space after = sign and the " symbol)
(Note, ext stands for the file extension you wish to remove from the association list.)
So for example, you wish to disassociate .jar files, so the command would be assoc .jar= "
3) Reboot the computer
Hope that helps.