Apple - How to prevent Command-I/Command-Shift-I from opening Mac Mail when in browser?

You can change the shortcuts from System Preferences:

If you want to disable the shortcuts, edit the NSUserKeyEquivalents dictionaries directly:

defaults write com.google.Chrome NSUserKeyEquivalents -dict-add 'Email Page Location' '\0'
defaults write com.google.Chrome.canary NSUserKeyEquivalents -dict-add 'Email Page Location' '\0'
defaults write com.apple.Safari NSUserKeyEquivalents -dict-add 'Email Link to This Page' '\0' 'Email This Page' '\0'

Then quit and reopen Chrome and Safari to apply the changes.

Taking a look at Chrome's keyboard shortcuts helps.

Cmnd+Shift+I is to "Email the current page". It's a bit annoying because it's so close to the developer tools (Cmnd+Opt+I).

The command-line solution given in another answer eventually worked for me but I needed to do one more thing:

after defaults write com.google.Chrome NSUserKeyEquivalents -dict-add 'Email Page Location' '\0', I entered the command: killall cfprefsd (and killall Finder for good measure).

This stopped the annoying Cmd-Shift-I shortcut on Chrome from creating a new mail message, but I can't get it to stop happening in Safari, unfortunately.

killall cfprefsd clears the preferences cache
killall Finder restarts the Finder

The GUI method described at the top didn't work at all for me, but maybe because I didn't clear the prefs cache.