how to stop a male cat spraying?

[CoolC]

We have 2 female cats, but there is a local male who comes in through our cat flap and sprays in the utility room.

Are there any old wives tales that will stop the little so-and-so?

Our cats are old and have never had collars so a magnetic flap is out of the question.

I've seen him in the room (glass door) but by the time I get there to shoo him away the little ninja (he's black) is out of the flap and dissapeared. Grrrr