The original Trek letter-writing campaign certainly had an effect. There were also some campaigns to save particular characters which had an effect -- although this also hinged on the other people involved in the production being interested as well.

In general, though, it's not going to have any effect on things unless some of the people involved in the targeted movie/show/etc. feel the same way as the fans. If none of them care, then you have no lever to work with. Your letters and actions have to have someone in the target area who listens... and who can then use your outrage to effect.