Hard - Impossible. China Already tried to do this in some capacity with deep packet inspection/ Only allowing HTTPs and such. People just circumvent this by just routing data thru HTTPS.

Ofcourse you can block IPs of popular VPN providers decreasing accessibility but it'd be impossible to stop it completely.

Though this would easily restrict the majority of non-tech savvy individuals while doing nothing

Wikipedia [1] documents some methods.

[1] https://en.m.wikipedia.org/wiki/Great_Firewall#Blocking_meth...