4
It's not preventable - if people want to break the rules, they will. However, many people are either ignorant of copyright or genuinely see 'borrowing' a bit of content as a fairly harmless crime.
If you let them know that you *do* mind with a polite email, a lot of people will apologise and take the stuff down. If that doesn't work, you could try making a complaint to their hosting company, who in many cases will be able to address such things through the terms of the hosting contract.
Doesn't always work though. Someone once sucked a lot of the publications off one of our work sites (which were available free for personal use), set up their own 'library' site with stuff (presumably) ripped off from many others as well, and then started trying to sell it. When we complained they barred our IP number !
Worst part - it was a supposedly reputable publishing house.