The Collaborative International Dictionary of English v.0.48:

Undestroyable \Un`de*stroy"a*ble\, a.
   Indestructible.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undestroyable
    adj 1: very long lasting; "less durable rocks were gradually
           worn away to form valleys"; "the perdurable granite of
           the ancient Appalachian spine of the continent" [syn:
           durable, indestructible, perdurable,
           undestroyable]
    2: not capable of being destroyed