In previous Windows version (i.e., XP and earlier) it was easier, now it's more complicated. But you can do it, two ways.
Easier, with third-party software:
download this and unpack. Run unassoc.exe, select file type from the list, and click on Remove association (user).
Still easy, but standalone:
go to registry and find HKEY_CLASSES_ROOT ,
then look out for the .extension you want to remove association with, and remove the (Default) value from here.