Violators will be fined 100 yuan. How do you say this in English?

Penalty means penalty; punishment; fine. If you want to write a slogan, you can use this word directly.

For example, Fishing in this river is forbidden; penalty ￡5 . Fishing is prohibited in this river and offenders will be fined five pounds.

Then your translation is Forbid the use of computers to do things non-work-related, penalty for 100RMB

You can also replace forbid with prohibit