I've seen people base64 encode their code and copyright notices, but if someone wants it bad enough they will get it. Your average Joe will have no idea they can just write a script to decode the code and just modify that into the current script, removing the copyright notice from the encoding, but is it worth it?

If someone has to go through those lengths, then so be it, they will and it will find its way to the Warez forums. Regardless, these are people that probably wouldnt pay anyways. Best to do it is to create information about the domain for each paying client, and when you find a site using your script, check your database for the license. Contact their ISP if using a warez script.

Or, if you wanna be really sneaky and dirty, and I don't know whether this would be a good idea or not ( though it would be funny and possibly teach them a lesson ), create a 'nulled' version of your script and write an exploit into it that wouldnt exist in the original. Catch someone using it and drop their site, making sure you can leave them a nice little message not to use nulled scripts. Would be embrassing for them to have their visitors see that.