The word "rectifiable" means capable of being corrected or put right. Its antonyms, on the other hand, signify aspects that can't be remedied easily. Some of the words that can be identified as antonyms of "rectifiable" are irreversible, irrevocable, irreparable, and irreversible. They all signify damage or changes that can't be corrected or undone.
Irreversible implies an action or event that is impossible to reverse, counteract or cancel. Irrevocable, meanwhile, connotes decisions, actions, or changes that once implemented, cannot be changed or revoked. Irreparable indicates damage that is beyond repair, while uncorrectable means errors that are impossible to remedy or fix.