Define illegal use: Using multiple copies on multiple sites? Reselling or giving it away? Using it to spam porn sites? One thing, if there are updates, and the user did not purchase from you, then no updates.
Put the script on a disc, seal it in a letter, get 3 people to sign over the seal, send it to yourself, it will be postmarked, therefore you have proof that you had it before the person who has stolen it. Which I guess doesent exactly protect it... but helps with sueing.