then it's very easy to script. For example you just put in some edit box and parse the input, which you compare with some algorithmic result and in case of inequality you call Game.Quit();
Then after successfull registration you write for example some value into registry or generate some file on harddisc with a key, which tells game, that it was already registered so it won't bother player anymore. This registry / file should be unique for installation or others will steal it.
This idea is not bulletproof but no protection is, so it's just another way how to annoy players.
Just note, that people who will steal he game won't buy it anyway, so using no-protection scheme saves you time and don't loose you that much money
Edit: morning typos