General synonyms:
* Rigorous
* Stringent
* Severe
* Disciplined
* Uncompromising
* Inflexible
* Tough
* Firm
More specific synonyms:
* Authoritarian (for rules and regulations)
* Exacting (for standards and requirements)
* Scrupulous (for adherence to principles)
* Punctual (for adhering to schedules)
* Meticulous (for attention to detail)
The best synonym to use will depend on the specific meaning you want to convey.