Thankfully, this sort of restriction (from a legal standpoint) is unenforceable. Licenses cannot be forced on a user who has already paid for the software in question. If I want to modify my copy of Microsoft Frontpage, and I've legally acquired the software, I'm fully within my rights to do so, regardless of what Microsoft says.
See: http://cr.yp.to/softwarelaw.html for a not-quite-up-to-date reference. A Federal judge a few weeks (months?) ago held up the unenforceablility of software licenses.
Matthew