When moderation blocks something a student has written, Mindjoy shows them a message. You can write your own, so it sounds like your school rather than like a system.
Setting your own messages
Click your organisation name at the bottom of the sidebar and choose Organization. You need to be an organisation admin.
Open the Moderation tab. Under Custom Responses, write the message you want students to see.
It is worth pointing somewhere useful, such as a named member of staff, rather than only saying no. The Moderation Policy above it (Strict, Moderate or Lenient) decides what gets blocked in the first place.