Oh, yes, the crowd. Joe Seidler came up with a "Spirit of the Sideline" which you can see on the UPA's web site. It says: "The ultimate sideline stresses encouragement and support. Cheering is encourage, but never at the expense of respect for all players and fans."
Now, think about any crowd you've been part of at an ultimate game. Is there the least bit of respect shown for any call, contested or not? The crowd automatically boos any call where there is the least bit of doubt. As someone who plays in front of crowds a couple times a year, I find this appalling.
(I think this is true only when there is a critical mass of spectators, enough so that an individual booer is anonymous. With smaller crowds, the individuals behave much more respectfully. They will still voice their opinion, but matter-of-factly. For instance, a player hanging out between games may point out of bounds on a close call, or comment on the merits of a foul call in a speaking voice.)
The worst example of this was at Worlds in Heilbronn in 2000, where we beat the Swedes 19-18. There was a foul call at double game point, and the Swedish defender incited the crowd with his unacceptable display of disbelief, then egged them on more post-game. But the crowd had no idea what the correct call was. I was standing five yards away, having thrown the pass, but I have no idea either. They booed the call immediately, they booed during the discussion, they booed in the aftermath, and they booed even louder when we were announced as the runner-up in the Spirit voting. They completely ruined the experience. I gave the post-game speech that day, and some who heard it probably thought that the reason I was so gloomy was because I felt that we won by cheating. Nothing could have been further from the truth, which was that I felt that the crowd and the Swede had stolen our rightfully-earned championship by their antics.
The crowd at Worlds in St. Andrews in 1999, in contrast, behaved much better. While it is probably true that they rooted for the other team mostly, presumably because they were the underdog having been seeded 28th or so, they did so respectfully, and gave both teams applause.
What can you do about this? Treat the players like you would if you were on the field yourself. These aren't professional athletes being paid millions. They are your peers, guys you see every tournament and who you might match up against soon. Be above the mob.