There are two groups of people that ruin bikes for everyone else. For the first group, we look to Bicycle Rights Extremists. The League of American Bicyclists is a group that aims to a) promote bike safety, b) encourage more people to buy bikes, c) encourage governments to make roads safer for bicyclists and d) spread awareness about ... like, bikes in general, I guess.
Not a lot of people know this, but being serious about biking means dressing like a dickhead.
They say things like, "Hey, bicyclists have a right to be on the road, so, car drivers, please try not to kill them," and they also say, "Hey, bicyclists: Having a right to be on the road doesn't mean you have MORE rights than cars, nor does it mean you're invincible, so please try not to be assholes." They're a good group, but, like all large groups and organizations, fringe subgroups are bound to emerge, and those subgroups will be full of idiots. Bike Rights Extremists, is what I've been calling them. These are the people who, as a "demonstration of bike rights," will clog streets up with bikers, making it impossible for cars to get through.