A word for something that can't be undone or undo ?
Synonyms for "permanent" from NOAD 2011:
lasting, enduring, indefinite, continuing, perpetual, everlasting, eternal, abiding, constant, irreparable, irreversible, lifelong, indissoluble, indelible, standing, perennial, unending, endless, never-ending, immutable, undying, imperishable, indestructible, ineradicable, ineliminable; literary sempiternal, perdurable, long-term, stable, secure, durable.
Synonyms for "secure" from NOAD 2011:
fastened, fixed, secured, done up; closed, shut, locked, safe, protected, sheltered, certain, assured, reliable, dependable, settled, fixed.
not a single word but
written in stone (or carved in stone/engraved in stone)
Fig. permanent or not subject to change. (Often in the negative.) Now, this isn't carved in stone yet, but this looks like the way it's going to be. Is this policy carved in stone, or can it still be modified? - The Free Dictionary
Like the definition says - it is usually used in the negative, but you can use it the other way for emphasis as well.