[BrianR]

I know a lot of evil things you can do to annoying neighbors. Most involve what could be called vandalism by authorities. The trick is to NOT get caught.

Try spilling bleach over an underground gas main, in a trail leading up to the house.. Spell bad things with it.

Does she have cable TV? Stick a pin into the wire and cut it off flush with the outer covering. That will kill the reception and be hell for the typical repair guy to find.

Mess with her car, a few BBs in a hubcap will panic a non-mechanical person.
If you can get into it, a clamp-on fishing weight on the accelerator cable will give her a nasty jackrabbit start. Dead roadkill inthe summer heat smells bad and will stink out the car forever.

You mentioned she keeps crap in her yard. Help her with her collection. Get other people's trash from the curb (legal to do) and store it in her yard, then complain about it. I once stuffed a dead fish into a neighbors drainpipe in summer, waited until the heat swelled it up and then the stink wafted over the area. Health Dept was called from several residences. Cats from all over moved into the yard. Mayhem and hilarity from me.

I have lots of ideas yet, pm me for more.

Brian