Locking the board without access to change the code.

I am working on a code for arduino using Inductive sensors. It is a simple code and not too complex.

The one thing I would like to know with arduino is that, is there a possibility to lock the microcontroller so that in case someone else has physical accessibility to the board, they should not be able to change the code that has been downloaded into the Microcontroller.

Physically locking the arduino in a case is possible, but is there any other way?

If the person can get access to the Arduino it is not really possible to stop someone uploading a new program. If you read the relevant Atmel datasheet (the Atmega 328 if you are using an Uno) you will see that you can set fuse bits that can prevent normal code uploading and can prevent the data that represents the program from being read. However with a high voltage programmer (which is easy to make) the program memory can be blanked out and the fuse bits reset.

What, exactly, do you want to prevent?

What level of expertise (or stupidity) are you trying to guard against?