I'm sure there's something out there that can help, however as jjasghar pointed out it's a shady area.
The reason that a trial version puts a registry key in place is so you can't just keep reinstalling the trial over and over without ever buying it, obviously. If there was a standardized, easy way of removing them, I'd think it would be illegal in some way.
Plus, different programs are likely to store their relevant keys in different ways in the registry. For some programs, you can just open the registry editor, do a search for the program name, and delete all the keys you find - other programs are smarter and store their keys in locations/with names that aren't so obvious and easy to locate.
Assuming the program you are referring to is of the not-so-simple-to-clean variety, perhaps if you called up the software manufacturer and proved that you had purchased a key, they might be able to tell you what keys to delete or change, or offer some other form of solution.