Depends on whether they're relatively soft wall tile, or a floor tile on the wall. Do you have any spare tiles? If you do, the safest and probably neatest is to tear them off and install new ones.
If they are soft wall tiles, then something like a RotoZip with a tile bit can cut them fairly quickly. To get a straight line, you'd probably want to put up a straightedge. If they are floor tiles, it'll be tough cutting a clean line. It will be very dusty, and harder to get a clean straight line. You'd need a drycut diamond blade, and though to do on the wall, or a diamond cup wheel.
Check out www.johnbridge.com to see if they have any other ideas.