@hfawwaz To use a hosts file, you create a file named "hosts" and in it you put a list of domain names followed by what IP address you want that host to resolve to. The networking software is designed to look for this file, and will skip the DNS lookup for any domain found there. So by setting an address of 1.1.1.1 or 0.0.0.0 you can prevent users from accessing a specific site.
There's a little more to it - obviously the file has to be where your OS expects it to be. Second, some malware tends to add its own list to any hosts file, so some anti-malware programs may want to remove it. You'll need to research that part yourself.
But with a hosts file, you could prevent users from accessing Opera's extensions site. Not 100% guaranteed to prevent them from installing extensions, but certainly makes it harder.