No, it's not really possible to prevent it. You can make it extremely difficult - some Starforce versions apparently accomplished that, at the expense of seriously pissing off a number of "users" (victims might be more accurate).
Your code is running on their system and they can do whatever they want with it. Attach a debugger, modify memory, whatever. That's just how it is.
Spore appears to be an elegant example of where draconian efforts in this direction have not only totally failed to prevent it from being shared around P2P networks etc, but has significantly harmed the image of the product and almost certainly the sales.
Also worth noting that users may need to crack copy protection for their own use; I recall playing Diablo on my laptop some years back, which had no internal optical drive. So I dropped in a no-cd crack, and was then entertained for several hours on a long plane flight. Forcing that kind of check, and hence users to work around it is a misfeature of the stupidest kind.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…