Strategies for addressing plagiarism once it occurs
You can have Student Services act as a buffer for you:
Often students will hound faculty members repeatedly and incessantly once a charge of plagiarism is levied. The Director of Student Services is the integrity officer for the School, and you are welcome to send such students to the Director, who will be glad to tell them repeatedly that they have to accept the grade they are given and to review what plagiarism is with them. If there are cases that are fuzzy, the director can also act as a mediator/witness in such disputes.
Useful strategies shared by other faculty (this list may grow as faculty share strategies):
One colleague has told me he used this at the beginning of the course he taught last fall upon encountering a few instances of plagiarism on an assignment. He told the class:
"I have found a certain number of cases of plagiarism. I won't tell you which ones or how many yet, but you have until midnight tonight to identify yourself with a reduced (or no) penalty. Those who I have identified who do not identify themselves by midnight will receive an F on the assignment." This uncovered additional instances of plagiarism of which he was not even aware.