If you have your file associations the way you want for a particular file type, you can remove it from the list that gets displayed in the Folder Options / File Types screen
1. Start Regedit
2. Go to HKEY_CLASSES_ROOT
3. Scroll to the file association you want to lock.
4. In the right panel, create a new Binary value
5. Call it EditFlags
6. Give it a value of 01 00 00 00
7. Now when you go to the Folder Options / File Types screen, you won't see that file type listed.