There are some posts about this in the
Hey, pennant guys thread
In my limited experience, there are 2 options.
1) Fill the bottom of a bath with warm water and add Oxiclean. Let it soak for 30 mins to an hour (paying close attention to ensure the printing doesn't start disolve). Gently rinse it in cold water and let dry.
2) Bleach: This needs a bit more care. Tape the pennant to a wall in your shower. Make sure to cover the spine so that the bleach can't get to it. Then put bleach in a spray bottle and spray the pennant. Let it sit for anywhere from 5 mins to 30 mins and then rinse thoroughly.
Both methods are not without risk. If it goes wrong, you can damage the colors on the pennant. It's probably worth trying it with an inexpensive pennant first.
Some consider bleaching to be the equivalent of trimming cards. Many others don't.
Others will probably be able to give more advice.