While offering an often desired feature, base/frameworks/notice/extend-email/hostnames.bro cannot be unloaded by users without modifying the scripts in base. Generally, it's considered a best practice to leave base alone.
I encountered this issue while trying to help someone determine why it was that they could not disable this script on the IRC channel.
I'd like to recommend moving this script to be loaded in the default local.bro (similar to how MHR is handled today) or adding a feature in the script for the feature to be disabled.
I'm more than happy to implement this and submit a pull request if someone can provide feedback on how this sort of thing should be handled.