Squirt water at it. My cat did this to someone who startet to be nice to it and now it makes itself at home in loads of houses and is a bit of a stray. If the idiot had discouraged it in the first place the cat might have learned its lesson and come home. Cats are fickle and will go anywhere they think is nice. I wish the said idiot had been as thoughtful as you and sent the cat away then I wouldnt have lost my cat and it wouldnt have lost a permanent home.
There are certain smells and such they don't like, you could try some oils such as citrus, peppermint, eucalyptus, lavender, ground chilli, coffee grounds or diluted surgical spirit.
1 cat
a shovel
jars of smelling salts
a water squirter
various smelly oils
jar of coffee
some chilli
bottle of surgical spirit
tiger poo
half filled bottles of water
1 dog
the original cat's owner