I once had a kind of a clash with a guy who was starting a forum which pretty much was head on competition to my own. The catch is that he was actually a moderator on my site so it felt like a kind of betrayal, but that was long ago.
The thing is though, competition is good. In Business Ideas Litmus Test Clay Collins actually counts there being competition in a given market as a good sign because it means that there is demand in that market and the market is already made so you wont have to build it yourself. You just need to be better at it than the competition and even if you're not number one you may still get more business than if you were trying to actually create an entirely new market (seek to do something where there's no competition).
Also, there's a very successful guy, a millionaire I think, but I can't remember the name, who said that all he does to succeed is look at what's already in a given market and then looks for ways to create something that would essentially cause a paradigm shift in that market. The demand is already there so all that's needed is a new approach, a new angle. People seem to be making a killing all the time by doing this and it can probably be done with forums, albeit it's not easy, of course.
Bottom line is that competition is good and may in fact be so good that it just wont be worth it quitting just because of few disgruntled competing admins. Maybe a relationship with them can be spun in a positive direction. Instead of bowing down it could be pushed more towards "friendly competition" type of thing. Show them what you've got and if it starts putting pressure on their business then well.. they'll just have to do better, wont they?
I think the recent improvements on DP have a lot to do with competitors like NB springing up to meet the former DPers.
Didn't mean to make it this long. Hope it helps.