There used to be this jerk of a neighbour. He had this big labrador dog -
and he would always go and let it defecate right in front of my front strip
lawn. Small dogs I can take - but he had a big dog that had big crap... And
when he kept on repeatedly doing it - what the hell is wrong with him??? The
crappy thing is that his wife would yell at other dog owners.

Thankfully, the bugger moved out - and hopefully is staying next to a Hell's
Angels gangleader who owns killer rottweillers.

anyhow, I had more important things to worry about - so I didn't let it get
to me... unless I sat down for awhile and thought about it.... moreover I
just didn't want an argument with the silly duffer. If I wanted to get even,
I collect his dog's feces from my front lawn - then throw it onto his front
door at 3am when noone is looking. If I was any younger - I'd collect my OWN
feces and hurl it into his front door step at 3am randomly.

I guess you could post a sign outside your lawn stating that you are using
an insecticide that is highly toxic to animals. Then casually warn passing
dog owners about insecticide - make up some story - about how it "may" cause
harm to their pets. You can also scatter highly hot chilli seeds onto the
areas when the dogs crap.

Warning! Insecticide Being Used may be Highly Toxic to animals.

Usually what they can't be certain of - usually puts an extra fear in them.