Heavily salt (whatever kind is cheapest) the yard, front and back. It will be impossible to even grow crab grass on it for a long time. Claim to know nothing about it no matter what and obviously don't leave a paper trail. I would avoid damaging the home for the sake of someone else who moves in unknowingly to broken pipes etc. Make the damage very very visual from the outside and unfixable. Heavily salting the lawn is the easiest thing I can think of. Even if the bank pays to put sod down, it will die shortly after. Nobody will want to buy and home with a dirt lawn and backyard. Cut down every tree and shrub on the property while it is still your right to do so. Put tar or something uncleanable on the windows. Sparsely stain the carpets. Dent concrete in the basement with a pic axe. Break the mailbox but leave it functioning. You can make the home valueless without the risk of an innocent couple getting screwed over. A big container of strong acid might have the same effect on the yard, and be harder to reverse.