Instead of using comparative and superlative forms, you can use other words or phrases to express different degrees of strictness:
More strict:
* More demanding
* More rigorous
* More severe
* More inflexible
Most strict:
* Most demanding
* Most rigorous
* Most severe
* Most inflexible
You can also use adverbs to modify "strict":
* Very strict
* Extremely strict
* Incredibly strict
Ultimately, the best way to express the degree of strictness depends on the specific context of your sentence.