I live in an apartment house and we have a yard in front of the house.
My problem is this: people that are walking their dogs are letting
their dogs go into our yard and leave behind unpleasant stuff. Because
my street is near some public institute, many different people are
walking there and there is no point in trying to educate them.
Also my neighbors would no like to build a fence. Is there a solution
that will turn off the dogs in some way? (Without killing the grass of
course...)

Ground pepper comes to mind. It's inexpensive, non-toxic and probably worth a try.

tutuzdad-ga

Yes, red pepper sprinkled in your yard will keep most way.  You have
to re-apply after each rain, though.

If your yard is big enough to avoid hitting people on the sidewalk,
you might try the motion-activated sprinklers people use to keep deer
away from their houses.

You might also consider simply putting up a sign that says "Please
clean up after your pets."

Hi inon,

I recently answered a similiar question.  There's quite a few
suggestions provided in the answer and comments.

http://answers.google.com/answers/threadview?id=326962

Best regards,
tlspiegel

Thanks very much for your posts - I will try these advice and will
report here my results!