I’ve read all the replies so far and I agree with @papyrophilia 's last post:
(emphasis mine)
You have their email addresses and IP addresses and they’ve confessed they’re the same users on both forums:
- Look up their Country, Province/State, city/town, ISP
- Compile a report worthy of sending to the local police department.
- Send this report to them privately, warning them that they’re just one click of a button away of being reported to their ISP and local police department.
- Emphasize that this is going beyond simple cyberbullying and that this is a group effort, thus cybermobbing
- Tell them that they should take down all and any copies of content from forum X on forum Y in a reasonable amount of time (24, 48, whatever hours. You know how much it’d take you to do the same)
- depending on the license you use for content creation on your forum / country you/they reside in you can threaten with the DMCA.
And if they continue their behaviour, update your report with the warning you sent and their new infractions and report them to their ISP and their local police department.
As you run a gaming site, and they don’t seem to be very mature individuals, that’ll probably frighten the **** out of them and they’ll cease and desist immediately.
If not, out of your hands, up to the 
…
