Spark's internal broadcasting option can be controlled through the Client Control plugin. If Broadcasting is disabled in this plugin, Spark will not even has broadcasting menu, also there will bo no broadcast menu entry when right-clicking groups. One would still be able to select multiple users, right click them and send a broadcast (though Spark users won't get such broadcast, only other clients like Exodus, etc.). But the option is "Allow for broadcasting of messages to specified groups or entire roster.", so it should allow broadcasting to separate users.
It does not matter if the plugin is not present or present but configured with restrictions. Spark ignores this and allows broadcasts regardless.