Emphasizing absolute absence:
* Not at all
* In no way
* Under no circumstances
* Absolutely not
* By no means
Emphasizing lack of occurrence:
* Not once
* Not ever
* Not a single time
* No time
Emphasizing duration:
* Forever (opposite of never)
* Always (opposite of never)
* Eternally
* Continuously
More informal alternatives:
* No way
* Not a chance
* Forget it
More specific alternatives:
* Rarely (implies very infrequent occurrence)
* Seldom (implies infrequent occurrence)
* Scarcely (implies barely any occurrence)
* Hardly ever (implies infrequent occurrence)
* Never again (implies an action will not be repeated)
The best alternative will depend on the specific sentence and what you want to emphasize.