I can’t really see it as trolling, for two reasons. One, it’s not about pissing people off but making a point. For the mods to sanction him, they’d have to admit that people shouldn’t follow discobot’s message, which is a good argument for changing said message. To me, the term trolling is reserved for those who post just to make people upset.
But, more importantly, welcoming posters is actually something we’ve been doing here for a long time. Sure, if everyone did it, it’s annoying. But it’s usually just one or two posters who say it, and then the thread goes on.
At most, I’d expect the mods to tell him to give other people a chance, or to try and include a response to the thread in addition to the welcome.