Hi, Englishsz. The idiom you want is not "fair cheating;" it's "no fair."
"No fair" is a very informal, colloquial way of saying "it's not fair" -- so the sentence means "It's not fair to cheat on that Spanish final (exam)" or "Cheating on the Spanish final is not fair."
It's mostly an expression children would use ("Hey, no fair! He got a bigger piece of cake!") or adults would use in very casual conversation.