A strange female cat is coming in your house and spraying what do I do to stop it?

Wait patiently for the cat with a high-powered water pistol or squeezy bottle full of cold, sterile water. When it enters the house give it a soaking, It won't be able to leave fast enough and is guaranteed not to return in a hurry! This is a perfectly humane deterrant and will not hurt the cat in any way. Since cats are very proud creatures, however, you will probably cause it to feel highly humiliated! (Another reason for it to stay away!) Alternatively, it is said that Lion faeces is the most potent deterrant to unwelcome animals. The smell alone is enough to keep even large dogs away! Of course, that is all well & good if you happen to have a local Lion faeces supplier! If not, go back to the water pistol idea; (This from a cat lover:) it really does work! There are also various sonic emitters available to buy. These keep animals away by making a continuous ultrasonic noise. From experience, though, they are not particularly reliable and have a tendancy to keep all animals away, not just cats. (Ok if you don't like birds, etc either!) Studies have also made potential links between inaudible noise and stress / irritability / migraines / insomnia.