Extremely dominant feature?
Rather than discarding the dominant feature (which will discard information), try reducing the number of features randomly selected when making each partition. In scikit's syntax this is max_features (mtry in R's randomForest). By default this is set to compare the square root of all features (http://scikit-learn.org/stable/modules/generated/sklearn.ensemble.RandomForestClassifier.html), instead try much smaller (perhaps even 2 aka decision stump). You'll probably also need many more trees vs. higher settings. This will allow you to extract information from more subtle features without losing all that great information provided by the dominant feature.