Joe, I wholeheartedly support your effort to stop people from infringing on your content copyrights.
As for licensing to PornHub, perhaps somewhere in the wording of the license you have grounds for terminating their license. Many content licenses put limits on the specific number of pix, or length of clips, which may be used for promotion in a non-members area.
Also, I'm not sure if you are aware, but other Tube sites can embed Pornhub videos on their own sites. Obviously that exacerbates the situation, which is not good for you (ideally, that is a loophole you can use to cancel their license, such as you were unaware that Pornhub was allowing your content to be used on sites other than their own - therefore, they should pay you for all of the other sites your content appears on or have their license terminated).
Otherwise, I know it's not the best option, but perhaps you could refund the money they paid for their license, and then terminate the license.
Good luck!
ADG