I dont use these traffic churning scripts but personally I would never buy into a script that didnt allow me to modify it if I thought (knew) I could improve upon it or customise it for my purposes.
As a coder I would have made the condition that if people wanted to modify it they could, but the 1% skim (Fuck it - Id set it higher
) had to be unmodified and the copyright link back to your site remain.
The most successful scripts are now developed in an open community environment where multiple people can add, bug fixes, additional functionality and plugins. Your program use terms would reflect that all 3rd party code can be integrated into the script without renumeration... As a "smart" developer you control the direction of the scripts development and simply choose which additions you want to use and add it to the next version you release.
You will *NEVER* stop people trying to modify your script.