Formal:
* Unchangeable
* Inalterable
* Permanent
* Unvarying
* Fixed
* Constant
* Steadfast
Informal:
* Set in stone
* Unmovable
* Solid
* Rock-solid
The best synonym to use will depend on the context of your writing. For example, "unchangeable" might be a good synonym in a formal setting, while "set in stone" might be a better choice in a more informal setting.