Social media can be a useful, even powerful tool for both consumers & companies alike. I've certainly used social media to resolve issues, and both gain & pass on important information. But like much of the net these days, it can & does get abused by the serial moaners, trolls & outright nasty people. I've witnessed how some users have repeated trolled & spammed the Twitter team at Northern and frankly I don't blame them for ultimately blocking some. At the end of the day, and as mentioned up-thread, the feed is not a requirement but an attempt to make an additional contact media available to passengers to pass on information & try to resolve live problems. The T&Cs are theirs and if users repeatedly break these then Northern are quite within their rights to simply block without comment (indeed I'm sure the same would be true on this forum!). Anyone that has run a forum or social media group will tell you that there are always people trying to be awkward, disrupt and even provoke, and so rules have to be in place to prevent these things getting out of hand.
As for the suggestion that Northern simply mute users / conversations, well as already explained they use a third party software package that allows them to manage all social media feeds, gather analytics etc. I've not used this particular one myself but have seen similar packages and the point of these is to be able to quickly respond (depending on demand) to queries & comments, keep easy track of ongoing conversations, and make sure that threads do not get out of control. And because of the latter the mute option simply isn't one you would want or use as a company or organisation. Northern, and indeed any company running media feeds need to be aware where threads are going off-topic, where conflict is emerging and indeed where inappropriate behaviour is occurring. Whilst it might seem to some like a good idea to constantly try to correct responses from the thread owners, in actual fact this is a form of trolling as such responses are not official and can often result in confusion for a passenger is seeking advice, especially where the third party advice is not entirely accurate. This is why constantly spamming their feeds with "advice" could lead to being blocked, just as you might expect to get thrown out of a ticket office if you constantly heckled the staff about selling the "wrong" tickets.