How do I set my external monitor so that my gestures won't activate spaces on the external?
Control over spaces and external displays is fairly limited in Lion. At the moment what you're looking for isn't possible.

One "space" is really the combination of your MacBook and external display, they can't be separated. Because of this, the number of spaces on your external display will be exactly the same as the number on your MacBook display, and you can't exclude one display from switching spaces or activating Mission Control.