Putting regular gas in a diesel engine, e.g.

• Banned action
• Taboo activity
• Taboo, to a toddler
• Mother's admonition
• Reprimand to a toddler
• Vigorous denial
• Ringo Starr's "___ Song"
• "___, Nanette"
• Words said with a finger wag

Wiktionary Word definitions in Wiktionary
n. (alternative spelling of no-no English)