In article , Nick Wagg says...

Once a spot has been fouled, it is more likely to be fouled again
because to a dog it smells like the sort of place where that sort of
thing is done.

I have heard it said, but cannot vouch for it, that if you use a watering can
and cover the fouled area with a solution of washing powder dogs are unlikely to
re-foul there. I don't know what effect this may have on your grass though.

-micko