I tried this and it works. But it is very much a matter of taste.
You can microwave them one at a time for just 15 seconds and they will craze all over. Then you can use them as coasters.
But do not use a hammer on them. You will get minute splinters going all over the place. Very dangerous.
Alternatively you can destroy all your data on them by scoring them with a Stanley knife and single line or a cross will do, then you can safely bin 'em.