Ability to broadcast message (not with chatbox) to multi users to take attention on something

We have 500 users, and there are many things happening like we have to inform/warn the user about something like virus spread, stop the thing to do etc. So we need a attention windows that can be broadcast to multiple users at a time. the chat windows can do the similar however the users can reply the chat and some dont know the situation and starts to reply and the things get complicated. So just an attention window to be appeared infront of the user at the middle like a warning message. This message can be text or jpg/png boosted. And yes for the offline machines, the screenconnect must assign a flag to them so that when they are online/connected the msg will broadcast at that time...