How to keep dogs off my lawn?

You can't do it. Put up a sign that says "Don't let your dog pee on my front lawn" and hope the people walking their dogs abide by your request.

You could probably get a small sign that requests it like this

I leave the pets/kids warning sign up for at least a few days each time when our spraying service comes but I've never watched the camera footage to see if it works

Motion sprinkler

My mother lives in a neighborhood of houses with very small yards. There is one house down the street that has a couple of the "Don't let your dog use the bathroom" signs in the yard. It seems to work, for I don't see any random crap in their yard, where I do see others.
But, of course this only controlled by residents walking their dogs. Obviously, dogs on the loose aren't going to read any signs.

. . . and you've neutered yours ?

Build a vet clinic on your front lawn.