inconvertible (adj.)

• changeless
• constant
• immutable
• incommutable
• indefeasible
• inert
• invariable
• irredeemable
• irretrievable
• irreversible
• irrevocable
• lasting
• noble
• nonreturnable
• nonreversible
• permanent
• unalterable
• unaltered
• unchangeable
• unchanged
• unchanging
• undeviating
• unmodifiable
• unremitting
• unsusceptible
• unvarying