Help the roommate give a crucial warning about the apartment's true ruler.

You ___ touch Mr. Fluffles while he's eating, unless you want to lose a finger.

The correct answer is had better not.

When giving strong advice or a warning, we use "had better." To make it negative, we add "not" directly after "better" (had better not + bare infinitive). "Would rather not" expresses a personal preference, not a warning.