How to set up in the spark or the Open fire server permissions for Berodcast messages that only dedicated users will be able to send it?
do you mean broadcast messages which are sent via the broadcast plugin or “client broadcasts”? For the latter there is no way as the client does send one message as a simple p2p chat message to every client. Of course one could block this but there’s help in doing this as a user still can send a message to every one - this would just take longer.
A simple policy which forbids to send broadcast messages may be sufficient.
If you have only Spark clients you may want to look at the client control plugin, it should allow to disable these broadcasts, at least I hope it does.