Well you can just succumb to the immaturity by leaving, but you know...just leaving is child-like as well! You don't see professional athletes leaving because some others are being unsportsmanlike! That was an example and I'm not comparing professional athletes to airsofters because they're nothing alike!
Moving on; be the bigger player and kick some ass! Show up and know that you will piss others off when you do kick their asses! If someone is ticked enough to go an verbally assault you and try to prove something that they couldn't prove on the battlefield, then that's just saying that you've done a good job, and you are better than that jack hole!
Maybe whiners and immaturity is caused by insecurity and trying too damn hard to fit in and/or impress! Maybe they're just pissed that they suck so damn bad! Sometimes the whiners whine because they've got away with it all their miserable lives without having someone there to slap some sense into them! I'll will let you all know that if you decide to approach me, star an attitude with me, I'll slap some sense into you and maybe even cram a period plug up your candy ass along with a glass of cranberry juice then send you on your merry way! Then again I have been known to be friendly at times!
Don't be tough guys, kids. It'll only get you hurt in more ways than one.