I think the correct command is:

firewall-cmd --permanent --add-rich-rule="rule family='ipv4' source address='180.76.15.0/24' reject"

Pay attention to highlighted part

Thank you xMudrii,

This command is executed successfully. Lets see if it works and the Chinese visitors from 180.76.15* will not have access to the site anymore.

Cheers,
Ivo

The command is not working, I run the command and reload my firewall for the changes to take effect, but unfortunately the 180.76.15* still have access to my site. Any other ideas?

You have to reload firewalld after adding a permanent rule.

firewall-cmd --reload