1. Making the older versions of electrum that are vulnerable to the attack obsolete or unusable for transactions until users are forced to get the more secure newer versions?
2. Make the download links of the older vulnerable versions inaccessible.
2. Before you can even get to that link, you'll see a big warning message on top of the download page:
Plus that direct link to the previous releases isn't endorsed in any other sites aside from forums/articles when pointing to old versions.
1. That "DOS attack" that has been mentioned, it does exactly that, it renders those outdated versions unable to fetch latest balance and broadcast transactions.
So the user might research or update to the latest version.
But the catch is: it requires the client to connect to a "counter-attacking server" to get blocked out of connection; not if it connects to a malicious server and non-patched servers.