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 Internet Explorer.
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.