Go to the key
HKEY_CURRENT_USER\Software\Policies\Microsoft\Internet Explorer\Control Panel. If this key doesn't exist, create it.
For example, if you only see
HKEY_CURRENT_USER\Software\Policies\Microsoft, then create
Internet Explorer as a subkey, and
Control Panel as a subkey of
On that key, create a new DWORD value called "Homepage" and set its value to 1.
Do the same for
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Internet Explorer\Control Panel.
This will disable the homepage options on the Internet Options dialog. You will see a message on the bottom of the dialog saying "Some settings are managed by your system administrator". It will also deny permission to webpages that attempt to change the homepage.
To re-enable it, set the Homepage value to 0.