[honest_george]

You can always sue of course (there is no union in the company). But what would you gain by doing so? They might be forced to hire you again, in which case you can be assured you will never receive a raise and will generally not enjoy working there a whole lot.

You could even try proving you worked too many hours, but that's quite hard to do (a list of start and end times would surely not suffice).

Edit: Also, they will surely claim that they did not tell you to perform that overtime, and that in fact you did so without them knowing.