No, you can't! 'Enjoin' is an unusual and very formal word which very few English speakers are even aware of. It is rarely used outside legal documents. Your friend obviously looked for a translation in a bilingual dictionary and found this completely inappropriate word. Your friend should have written something much simpler, such as "He told them that if they did that again, they would be grounded for 2 weeks."

Hi! I am a native english speaker and have actually never heard someone say the word "enjoin". Looks like it could be used mainly in legal settings? I would probably use another world like "explained" or "warned".