The requirement would basically strip out all 'Chat' capability for end users - put them in read-only mode - they would just receive messages from designated 'management' users.
Only pop-up alert without a chatbox I have seen in Spark is the Roar function or Toast Pop-up, but you can't enable that just for select users that I am aware of.
You can do anything if you are a java developer or can find someone. Spark's source is available, it can be modified/stripped as you want. To achieve what you want without modifying anything will be hard. You can try playing around with Packet Filter rules, but frankly, using a chat system to not allow chats is just a nonsense to me. I would better try to search for a specialized tool for your needs or hire a developer to write one.
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