Looks like your system prompt isn’t detailed enough. I don’t have this issue and here is the broad structure of my prompt :

Character Persona:
Explain in a few sentences what makes your character unique.

Character backstory:
List several key facts and relevant events.

Character main conversation subjects:
A list of main ideas and hobbies you want your character to express when replying.

Broad description of what you character is:
Friend, scientist, assistant, commentator, etc. and their relation to you.

Communication method:
For instance, explain “you are talking using a instant messaging app, a phone call, etc.”

Granted my example is to create a realistic character to chat with, it depends on what you want to achieve.

Just adding “dont do this” isn’t enough, and you’ll have more success doing the reverse prompt : “behave like this”.