I see what you're talking about constantly, Murdr. This is the single biggest difference between "old-school" people and the "new blood". The old schoolers kept their lip zipped until they had some skill worth talking about. Nowadays, it's all in their head. All the time you see a "nobody" talking smack to a "well known" highly skilled player. The no name guy invariably has 0.368 K/d and 0.279 K/S w/ 1 kill every 90 minutes.
The only success I have had is to give them perspective via statistical dissection of their effectiveness. Basically, if a person can't get a 1:1 K/D and K/S w/ 4+ K/H they just aren't effective, and certainly have no basis for talking crap to anyone. It's not about score even in the slightest, it's just about being effective. If you are ineffective tour in and tour out under no circumstances should you be badmouthing people who are. You should be listening, asking questions, filming and working on stuff. Wasting energy distracting yourself on 200 with baseless chest thumping and smack talk is just going to make that person even less effective than they already are.
However, I will say, just because someone is "ineffective" in the MA does not necessarily mean they are completely skill-less. They actually can have a lot of skill in one particular facet or another. What is often the case is that they are almost completely deficient in one or more key areas. This is the source of their frustration. But, calling their attention to the fact that they are actually, in the big scheme of things, totally ineffective, while somewhat harsh at that moment of revelation, often acts as a catalyst to initiate a rigorous program of self-improvement and they tend to stop the smack talk on 200 for the most part.