Probably the easiest place to wire a disable switch in is at the clutch interlock (the switch that tells the car the clutch pedal is fully depressed).

important note: always break circuits on the low power side of the relay in the circuit.

-Go to RadioShack buy an 1/8 mini plug extension cable (42-2472, around $5).
-cut the male end of the ext cable off
-solder and shrink tube the two wires together on the male end (making a loopback)
-wire the female part into the clutch interlock switch so it breaks the circuit when the loopback is removed
-put the female end where ever you want
>>result: starter won't engage with loopback plug missing (push start possible)

A thief might find a hidden switch. But, an empty female mini plug connector doesn't look like a switch and without the loopback there's no way to complete the circuit.

Other things you can do:
-use the same method to cut the low power connection to the main relay
>>result: car won't do anything with loopback plug missing.

-buy and use a clutch/brake pedal lock (around $50)