I was in a freerun lobby on Daytona and there was a Spaniard using Jay Leno's tank rod, trying to block the track and ram people. I'm not sure if he ever got anyone.
As for the American issue, I've heard a lot of noise about it but haven't really noticed Americans acting like idiots.
I think part of the problem is the high standard of living, thus, more parents can afford to buy expensive equipment for their kids. They then give their kids unsupervised access to the PSN. Guess what happens next.
Alcohol may be a factor in other cases.
The others, I guess, are people with big egos and little talent who don't really see any value in clean racing. As many have said, these people exist in all countries, but since America has more population, they'll seem to have a higher concentration of non-smart people.
From what I've heard, the British can be far worse. Two UK users in one lobby instantly becomes an argument of the "my soccer team can beat up your soccer team" type. I don't have any firsthand experience in this area, fortunately.