First, you make it difficult to reach unapproved sites, necessitating a VPN.
Then you discourage use of VPN's and prohibit advertising thereof. Not illegal, just hard to find and maintain.
Then you block search terms for VPN's and workarounds from the approved search engines.
Then of course you block the official, and mirror, VPN download sites.
And then you disrupt connections to the VPN's external server locations, either block completely or throttle to a slow crawl.
It's a constant game of VPN's adding servers and mirror sites, and the government adding more bricks to the great firewall.
Not always as easy as sending an email. Even with constant upgrades and patches and new servers, the VPN's can't manage to keep up with the new tricks during party congresses. So that's a couple weeks a year that it's nearly impossible to use your already installed and running VPN.