What do you do when one of your moderators or one of your valued members departs on good terms (or even bad terms) and starts a forum that is competing with yours? What do you do if they start recruiting at your forum for new members for their forum?
It is never pleasant when this happens; of course you are not going to be happy about it when it happens; and there is not a lot you can do about it. There is no use having a dummy spit over it.
It has happened to me a number of times. The only effect it has had on me is that it just motivates me to make my forum even better. Each thread is treated more like a baby and nurtured. Breaking news for new threads is always sought after (it can get interesting when a new forum uses your forum as their source for new threads – members of both forums are not silly and can see what is going on). New ideas are tried. The content is kept fresh. The link building is kept up, so I dominate the search engines rankings to keep new members coming in (how is that competing forum going to go after they get initial members from your forum – where are they going to get more newbies from? The search engines?). Try and dominate the search engines to make it harder for competitors to get established (I use Linkvana and Article Automation to build lots of deeplinks to individual threads, so they all get indexed and rank well for the short and long tail searches).
Let the members vote with their feet. The forum that best meets their needs will survive. Maybe you are not meeting your member’s needs, so keep a close watch on your metrics.
Perhaps it is important to have a clause or something similar in your forum rules or terms of service about not posting messages or using the private message system to recruit members for another forum. I have never actually enforced this rule and left messages there, in case I get accused of sour grapes. However, if they start abusing the private message system, you may need to act.
Do not get in petty squabbles about which forum is better. Let them do that. Delete any threads on your forum comparing yours with theirs. It will only hurt you in the long term.
HOWEVER, also use this time to do an analysis, some hard thinking and take on board any criticisms.
Guess what? Every time that this has happened to me, they failed! Use it as motivation. The best way to get back at them is to have the members vote with their feet because your forum is better. Ignore them, make your forum better!