As soon as I posted my previous, tagged to Randy, I realized that I had used a generic 'you...'(it's a rotten thing to do to another teacher in your building) that in the context of an offlist accusation could have been interpreted as flaming. Quite the opposite of what I intended. My apologies. I have never been poached but a form of poaching has happened in my district. Let me explain:
Two of us middle school teachers have started to teach the pantyhose sculpture lesson. As you know, this is one that several of you in this very group have posted, with instructions, so it's out there, pretty much public domaine. The two of us who do it approach the thing quite differently. We have a colleague, the third middle school teacher, who was planning on trying it. But in the last district art show, one of the second grade art teachers whose students feed into that third school displayed some. The art coordinator, who'se new and did not know that this was a middle school assignment, thought they were wonderful and when I pointed out that this was supposed to be a middle school lesson, I don't think it sunk in.
There are of course, ways around this. In our case the lag in years between 2nd and 7th grade is a long one so you could simply tell the class, "well, you did it then as a 2nd grader and now you're going to do it as a 7th grader". Or use the incident for a class discussion lesson about copyright and plagiarism (I'm not sure that even I would have the nerve to do this if the offender were at my own site).
One more thing: there is a difference between the concepts we teach (contour drawing, value shading, color mixing, clay joining etc) and the actual assignments that realize teaching these concepts. Concepts should be repeated year after year and built upon (do math teachers teach fractions and then move on?) but there are assignments that can only be done once. Remember, the kind of theft Randy and others have been talking about even includes swiping and making class copies of handout sheets.
If this is happening in your school, your department is not functioning as a team. I would think that getting together as a department and mapping your curriculum would address the issue. It may even be that your poacher is not venal, just obtuse.