Since you have to expose your code to your customer. It seems there is NO real technical solution. So Create two copies of your code(Your copy and Customer's copy). Your copy should be well commented, well indented. And in Customer' copy
1) Convert complete code to a single line
2) Remove all good comments
3) Insert wrong comments, which would add to the confusion.
4) define non-sense variables with some mis-leading names, subroutines/classes/Modules with some mis-leading names and don't use them in code.
5) Increase size of the code by writing non-sense comments.
6) Take care that these things would NOT affect the output by any means.

Most probably customer would prefer to give you maintainance part rather than debugging your code. However a real debugger would still extract your logic, but you can always make his life difficult.

Regards,
Sandeep