12 guage comes to mind...
...take a few cuts off the plant, pop them in a cloner and see if its anything special.
Either way, a person who would be so brazen as to plant it right there in plain sight is the sort to be vindictive and spiteful. Just exactly the sort who causes headaches if they get the idea that somebody ripped their crop, and they know who it was...quote]
IMO this thought is on the money. Short story--A local landscaping truck pulling a trailer decided to turn around in my neighbor's driveway. He got all cockeyed and ended up giving them an accidental lawnjob (rainy season) but then backed into and knocked over their mailbox. Then the guy just drove away, but before he did he looked right at me watching out my window. Without thinking much about the guy seeing me, I called my neighbor with the phone number of the landscaping company, as it was in big bold letters on the side of their truck. One week later a very large patch of prime zoysia grass in my yard was dead, right in front of the windows we were looking out of. I couldn't figure out at first why my grass was dying, then it dawned on me that I had received a landscaper's version of thug justice--I'm pretty sure the clownboy spray-tanked my grass with roundup for calling him in.
Two days later the cops show up asking why two dead Mexicans said they got ill from your agave.