Apart from what OriginalGriff said, you can maximum try to set license to it and make it little difficult to use illegal copy.
There are different ways to achieve this:
1) encrypted license files that contains the expiry dates etc.
2) encrypted license keys held in the registry
3) challenge/response mechanism which require an internet connection
4) probably others.......
Try:
License Key Generation[
^] or:
How can I create a Product Key for my C# App[
^] or:
c# Licensing[
^]
Also,
Cryptography 101 for the .NET Framework[
^]
Hope this helps.