Note that this is a vicious cycle as each BackRoomDecision separates the cabal from the newcomers and the newcomers from the cabal. A short term Pyrrhic victory is to LimitGrowth and pull the community back across TheTippingPoint until it can figure out what to do, but this will cost you a lot in the long term. The RightToFork may help as well, and you might experience an explosion in forks (as WikiPedia did), but this will often kill the mother community by splitting its top talent (the fork leaders) and splinter the daughter communities (from animosity), thus ending the entire enterprise (cf. ForkingOfOnlineCommunities).
A partially better answer may be to engage as many newcomers as possible, form personal relationships, and mentor them (a la Brainstorms' buddy system), but this might only push the snowball faster down the hill. At a certain point, the diameter of the social network is too large to sustain BarnRaising and it requires formal management, at which point the project descends into politics that will bore away and burn off your core talent who are only there for the joy of BarnRaising with other similar people. After all, there is nothing holding people to your community except your personal relationships with them, and drowning those out in an overcrowded public environment will essentially be death.
Additionally, PeerPressure becomes weakened as peer pressure between strangers is weaker than between friends. That is, the threat of shame is stronger amongst friends than strangers (cf. GroupThink). To think of it this way, how do you DissuadeInteraction with someone who doesn't care about your opinion of them? If people are only here for IncidentalCollaboration with the community rather than BarnRaising, they will not mind if you do not like them as they will stay anyway. This also leads to trolling (cf. WhatIsaTroll) and AdoringFans who are only here to be near the abstract social energy, not for a specific friendship. LimitGrowth to LimitTemptation only works as long as you are successful, but once the genie is out of the bottle, you are now a tempting target for attackers and for those who want to be in the warmth of where "it's happening". Kill the cool hunters if you can, but you won't be able to do this for too long.
Another possible solution is to MisdirectTemptation?. Wiki:BigWikiFireOfDoubleOught was an interesting concept where the newcomers were given an abandoned copy of the original site whilst the oldtimers kept going with their own fork. In essence, "StartAgain!" they cried. This is a waste of energy though. Another option is to become a GatedCommunity, either through HardSecurity or through psychological means (being excessively elitist), but the former will kill your community and the latter will sully your reputation. Another is to give up on your old values and accept that you have transformed into something new and enjoy it, but if you prefer ContentOverForm, that won't really work for you. You may have to exercise your RightToLeave. You can also try delaying the situation by burning off as much of TheCollective as possible and building a whole new collective (e.g. Wiki:MissingWikiBeforeXp). This pulls you back down the life cycle for awhile, but also exacerbates the problem as now you have two communities that are pulling people in, and you have created an intrinsic SeparationOfPositions? you won't be able to escape.
One of the GreatChallengesToWikis.