*Not official legal advice:
Registering the trademark probably gets you nowhere with this. You still have to gain popularity with the brand etc. and the mark has to be known to consumers etc. A lot of things go into making a trademark protectable IP and the registry really isn't too high on the list.
In IP, one of the cases we read involved the University of Texas' Longhorns symbol. A small company was using the same exact image (different colors), and the court ended up finding that there was no trademark infringement b/c the Longhorns association w/ the symbol wasn't strong enough (the court relied on surveys of the general population).
Trademarks are one of the weaker areas of IP law. You can't really trademark something that doesn't exist yet. There has to be a strong association between the trademark & your brand etc. & there's several factors to weigh when 1) determining if the trademark exists & 2)determining if they have actually infringed. Also, if it's just a name, it'll most likely be harder to prove a trademark, especially if it is a descriptive name etc.
Oddly, if they're copying the content from the subreddit, and it's your original content, you can probably make a legitimate copyright claim.