Feature Activation Logic (Prevent Activation)

Question

I have three site collection features used to set branding throughout the site collection. I want to prevent the accidental activation of one of the features if one of the three is already activated; otherwise they'll override each other.

I'm assuming that logic in the FeatureActivated portion of the feature's event receiver is the right place to put the logic in, but I'm unsure how to do that logic and return a message to the user saying that the activation was prevented because there's already a feature activated.

Any thoughts?