Here you can configure Prosody <a href="https://modules.prosody.im/mod_firewall" target="_blank">mod_firewall</a> module. You can create multiple configuration files bellow, and change their order. Don't hesitate to share your configurations with the community (for example by adding some examples in the plugin documentation).
Warning: mod_firewall is disabled in the livechat <a href="/admin/plugins/show/peertube-plugin-livechat" target="_blank">pluginsettings</a>, you have to enable it if you want this configuration to be taken into account.
If you can't make it work, you can use the diagnostic tool (there is a button on top of the pluginsettings page), and take a close look on the «Prosody check» section.
You can enable an "Emojy only mode" in your chatrooms. When this mode is enabled, participants can only send emojis (standard, or channel custom emojis). Moderators are not affected by this limitation.