The "cheat" way to do this is to change the value
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionInternet
Settingsones1609
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionInternet
Settingsones11609
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionInternet
Settingsones21609
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionInternet
Settingsones31609
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionInternet
Settingsones41609
Where 0-4 are Zone identifiers and the value is 0 to Allow, 1 to Prompt, and 3 to Block. Keep in mind that if your code does this on anyone's machine but your own, you're likely to find your code blocked as malware.
The "proper" way to do this is to use the APIs to create an IInternetZoneManager and call SetZoneActionPolicy to adjust the settings for URLACTION_HTML_MIXED_CONTENT in the zones you want to adjust.
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