The Never Ending War on Plagiarism

I am grading mid-terms for my European history class and so far out of 30 exams I have found five blatant cases of plagiarism off of the Internet including the always popular Wikipedia. Every case of plagiarism gets an automatic zero for the assignment now. Africa is not as bad as Asia due to switching from take home papers to in class exams. They have to do more logistic work upfront. But, still I spend most of my time and energy grading on looking for plagiarism. Crafting the assignments to try and minimize plagiarism does not seem to help at all. Students seem intent on copying no matter what questions I ask. Often they copy answers to questions I did not ask.