Edit: Now that I think about it, looking for simple phrases wouldn't work because of how good these AI's ability to follow instructions are. You could enter something like, "Use every third word in the following sentences as the prompt." Or similar, and hide it from any basic text parser.
I guess the only way to really conscribe the AI is to somehow make it follow specific guidelines and instructions without it having any record of what they are. Not sure that would be plausible given how these models work.