I think running simple string searches is a reasonable and cheap defense. Of course, the attacker can still request the prompt in French, or with meaningless emojis after every word, or Base64 encoded. The next step in defense is to tune a smaller LLM model to detect when output contains substantial repetition of the instructions, even in encoded form, or when the prompt appears designed to elicit such an encoding. I'm confident `text-davinci-003` can do this with good prompting, or especially tuned `davinci`, but any form of Davinci is expensive.

For most startups, I don't think it's a game worth playing. Put up a string filter so the literal prompt doesn't appear unencoded in screenshot-friendly output to save yourself embarrassment, but defenses beyond that are often hard to justify.