General:
* Inevitable
* Irresistible
* Unchecked
* Indomitable
* Invincible
* Unbreakable
More specific:
* For a person: Tenacious, Determined, Relentless, Resolute
* For a force: Unyielding, Overwhelming, Imminent, Uncontrollable
* For a process: Irreversible, Unhindered, Unbreakable
The best synonym will depend on the specific situation and the nuance you want to convey.