I'm sure I saw a tip a couple of weeks ago that explained *exactly* how you can do this. Essentially, each app has a preferences file in it that tells the OS what files it would like to open. However, you're hardly likely to use QuickTime player to open JPG files, so it is worthwhile being able to take off the associations you're never likely to use.
I think I've worked it out, but it is a bit fiddly. You need to have developer tools installed (so you can edit the PLIST files). Then, for each application, just right click on the application's icon and choose "Show package contents". Double click on "Contents" and then "Info.plist".
This opens that application's specifications in the Property List Editor. Now, navigate down to Root --> CF Bundle Document Types, and you'll find an entry for each document type the app can open. The extension for this type is "CFBundleTypeName". If you don't want that app to open that type of document anymore, just delete that entry.
After a reboot, the changes you've made will take effect and you will no longer be offerred Quicktime as a choice for opening BMP files, and so on.
Like I said, fiddly, but it can be done.
EDIT: PS - Do not forget to backup the app before you change it!