There is virtually no way to copy protect an isolated program (a single player game, for example), because by the nature of computer programs they can be modified to "think" they are valid, when they're not. For example, even if your single player game connects to a server to verify your unique CD key hasn't been pirated, all it takes is a quick change of your EXE and now your game will always think it's validated by the server.
Like ImaginaryHuman said, the only way so far that's successfully deterred piracy is multiplayer validation where the server - an external copy protection check - will kick you out if your codes are not valid. This way no matter how much you modify the code on your end, the server will refuse your multiplayer connection if you don't provide the correct access codes. The only way to bypass this would be to hack the server.
In other words, people are free to do whatever they want with their own PCs, because they have full control over the code that executes on them. The only way to forcefully stop the player from illegally playing a game is to deny them a connection to some online service.
This means pirates can be blocked from dynamic multiplayer and online content, but they cannot be blocked from content on their own computer, or static online content that can be copied locally (such as validation data).
Like ImaginaryHuman said, the only way so far that's successfully deterred piracy is multiplayer validation where the server - an external copy protection check - will kick you out if your codes are not valid. This way no matter how much you modify the code on your end, the server will refuse your multiplayer connection if you don't provide the correct access codes. The only way to bypass this would be to hack the server.
In other words, people are free to do whatever they want with their own PCs, because they have full control over the code that executes on them. The only way to forcefully stop the player from illegally playing a game is to deny them a connection to some online service.
This means pirates can be blocked from dynamic multiplayer and online content, but they cannot be blocked from content on their own computer, or static online content that can be copied locally (such as validation data).