How do I dissociate a program from a file type?
In a discussion I found, a Microsoft MVP recommended the use of this program, "unassoc.exe", perfect for your needs.
unassoc is a small portable executable, whose only UI elements include a list of all file types with associations on your computer, the ability to select one or more extensions, and the ability to delete the file type associated with that extension.
Also, if you're simply worried about the overhead of having multiple file extensions registered to a program you don't have installed anymore, the registry cleaner bundled with CCleaner can automate this process for you. It can scan for unused file extensions, and delete the relevant registry entries.