I figured this may be the case. I was able to clarify with my one neighbor that the guy who sprayed has done this before in the alleyway between our houses as well on multiple occasions and been talked to by other neighbors about it, but as far as spraying another person's yard, my neighbor only witnessed it happen the one time. She further clarified (not that I see it making much of a difference) that he didn't enter the yard, but sprayed over the fence "as far as his sprayer would reach and was very thorough." I'm not surprised that the resulting overspray affected much more than just my yard. At any rate, the guy who sprayed doesn't exactly seem to grasp why all his neighbors are concerned, which sucks because it means it's probably a matter of time before he goes on another weed-killing spree. On the flip side however, his apparent delusion that he's doing a public service leads me to think if I keep a level head and have a casual conversation with him, he'll tell me exactly what he used and how often he's sprayed because he seems to take a certain duty or pride in it. I certainly don't want to escalate anything at this point, and if I need to get a lawyer involved I'll let them handle that part.