i'm sorry guys, but i must say the suggestions of putting various things in with the pot is extremely stupid, potentially harmful (to them and maybe even you), and childish.
the way i see it, the weed is not on your land, so it is of no concern to you. the fact that they are trespassing on your land is the only real problem - string some fencing wire or barbed wire on your border, along with some signs. i personally would not leave a note saying that you found it - letting them know you know about the stash might frighten or anger them, OTOH, they might just hide it somewhere else, but it's a gamble. and if you say they're trespassing on your land to stash it, they'll know who you are. the weed is not yours, and thus not yours to destroy or give away...i might be inclined to "sample" it, but that poses risks to me - i don't know where it came from.
i am in no way opposed to weed or the smoking of it, so i suppose i'm more inclined to leave it alone than someone who thinks weed is bad. stoners are a gentler breed of drug users, most i've met are calm, nice, and generally pretty smart. i would just deal with the only issue that really concerns you - the trespassing of your land.
tmm