You mean on just one side? Try a hacksaw, it is just aluminum is it not? Make sure you stick a fan on the heatsink afterward though.

If you do cut it, make sure you look at it from the side once mounted to check for gaps between the die itself and the contact area of the heatsink. It is okay if the spacer around the die is not making contact, that just helps prevent the die from being crushed if the heatsink is not sitting flat.