stray cats bothering our cat

Question
HI, we are living in 'n townhouse complex in Windhoek, Namibia.  We have lots of wide open spaces and currently have one cat.  Everything was fine until a month or so ago, when a stray cat started coming into our house (through the cat flat) and eat our cat's food and usually a fight then start.  this is quite disturbing as this usually takes place late in the evenings when we are sleeping.  unfortunately our cat is not able to chase the other cat away and is now to afraid to go outside.  we now started to close the cat flap at night, but this is not good for our cat as she was used to going out at night to the garden.  Can you please help with any advise??  
thank you
Daleen

Answer
Hi Daleen,
The fighting is likely due to the other cat, or yours, not being "fixed".  If you get them both fixed they shouldn't fight so much.  Otherwise in order to keep the other cat out you will have to try putting the food bowls outside.  I don't know of any other options for you, this cat has found your home and likes it there.