Contact the forum operators directly (it's up to them how they choose to do this - anything from politely ask them to purchase a license, to strongly imply legal repercussions).
Issue a DMCA takedown notice to the hosting company of the forum.
File for injunctive relief and/or compensatory damages.
Each option has its own set of upsides and downsides.
Often times the people who pirate this kind of (very cheap) software are people who genuinely can't afford it. It can be beneficial to ignore these guys as they are still increasing your market share and popularizing your platform. And often times they will remedy themselves and eventually purchase a license when they can afford it.
Also, issuing DMCA takedown notices are often of little effect to the kinds of people that pirate forum platforms. Chances are they're going to ignore it, and if forced, simply change to a more "piracy friendly" host outside the influence of DMCA, rather than actually shutdown or pay for a license. And with more and more sites protected by reverse proxies, it can be difficult to even identify the host these days.
Being too hostile to such people can be far more detrimental than beneficial as it creates, rightly or wrongly, a feeling of injustice/oppression that can disenfranchise people toward the company, rather than endear them. As a company it's better to inspire affection than to impose obedience. XF does this well.
Finally, legal action would only make sense for the most egregious of violations. Sites that are actively cracking/reselling Xenforo, or otherwise making a ton of profit from the XF brand or XF software. However, there are also jurisdictional considerations here.
While it's bad business and legal practice to comment on action being taken. I generally see another reason. The only good information they could provide basically includes limitations in their ability to persue. Which is free info on how to avoid takedowns. Not something they want to provide.