Make it look like a prototype? I've taken to plastering big warning on non-production systems. Otherwise someone looks at a fragile dev environment, doesn't see it breaking outright, then goes and sells live customers into it and uses the prototype/demo system to setup real accounts. No amount of verbal or email warnings will change this. Hurray, now the one-off demo system is a production environment! As a bonus, you get to write data migration programs to move these customers to the entirely-unrelated final product!

But instead, if there are big red Xs and so on in the UI, then it becomes clear "oh this is a prototype/demo/".

Instead of explaining it, there can be steps taken to ensure a transition between prototype and product, such as prototyping in a language or for a stack that your company does not use in production.