<p>Two thoughts come to mind.</p>
<p>You could, conceivably, use a boot CD to access the domain controller while it’s offline and manually edit or delete the offending GPO - a domain’s GPOs exist under the <a href="https://support.microsoft.com/en-us/kb/324175"><code>SYSVOL</code> folder</a> in the file system on domain controllers, and are applied as registry settings, both of which are accessible from a Boot CD - however, this would either be undone by replication or would cause domain replication errors as soon as the domain controller you did this on connected to the other domain controller(s) in the domain. (I’m making the assumption here that you do have more than one domain controller in your domain, as you should… if you only have the one, this wouldn’t be a bad approach).</p>
<p>The other approach that comes to mind is to enter <a href="https://technet.microsoft.com/en-us/library/cc794729(v=ws.10).aspx">Directory Services Restore Mode</a> and perform an authoritative restore from a backup that predates this GPO. (And this too, relies on the assumption that you’re doing as you should do, and have backups to restore from.)</p>