Has anyone got any tips for (humanely) deterring a cat that keeps on visiting your house?

We have a cat already, and this one belongs to a neighbour. Unfortunately it's hard as nails and terrifies our own. At the moment we've got a water pistol with which we escort it from the premises (!) but every time I get in from work it's back on the sofa, while our own cat is hiding upstairs. Any advice on how better to put off the visitor would be much appreciated.