Emails and SMS
As with most messaging services, there is no way to prevent guardians from replying to emails or SMS messages.
You may want to use a 'no reply' email address to send from, send out a communication to let them know you would prefer them not to reply, or include a reminder in the email or SMS not to reply.
You can see further details on what happens when someone replies here: What happens when someone replies to an email or SMS?
It is possible to prevent guardians from replying to in-app messages.
This can be done from School > Communications > Setup > In-App Message Settings. Click the Accept messages from guardians setting and untick the box.
Please note that this will prevent guardians from sending any in-app messages as well as stopping them from being able to reply to your in-app messages.