Keeping Dogs off Grass

[elplews]

Hi, Does anybody know of any products or anything else that will stop a dog coming into my garden and using it as a toilet!   Its only a small patch of grass that I have at the side of my house (for different reasons am unable to put a fence/wall up)  Obviously I don't want to harm the dogs, just stop them from making a mess.   If it could be for dogs only that would be great, (I do have a cat) but if its a dog/cat thing combined then so be it......my cat will have to play elsewhere!