Thank you for clarifying. Unless you have an IP firewall set up in the User Profile to block this traffic, we wouldn't be stopping these connections. I did find this article that might be helpful- https://social.technet.microsoft.com/Forums/ie/en-US/46667a11-5a49-4549-9e18-7d07e8001c35/error-789-when-trying-to-establish-l2tp-vpn-connection?forum=w7itpronetworking
If you're trying to block those VPN connections, you'd want to identify what ports they are using and block their traffic on those ports. I'd recommend using your main internet facing firewall for this.
