As I read in the "Government use" section in the VPN blocking article on Wikipedia. It seems that Iran and China (used to) block access to use VPN software. I'd like to widen the scope of the question and apply it on any type of software, not specifically VPN or encryption software.
On what grounds can a government delegalise the use of specific software? And is delegalization of (specific) software compliant with international laws (human rights)?
Governments have power to do whatever their constitutions (written or unwritten) allow them to do.
For example, the constitution of Australia provides:
The federal Parliament can make laws only on certain matters. These include: ... post and telecommunications; ...
The telecommunications power covers VPN and any software that uses the Internet for delivery or communication (i.e. virtually all modern software).
In addition the federal government has power over inter-state and international trade (any software that crosses state or international borders) and corporations (any software made, sold or used by companies).
If they want to ban a piece of software they have pretty strong constitutional power to do so.
