Locking File Associations

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
Start Regedit
Go to HKEY_CLASSES_ROOT
Scroll to the file association you want to lock.
In the right panel, create a new Binary value
Call it EditFlags
Give it a value of 01 00 00 00
Now when you go to the Folder Options / File Types screen, you won't see that file type listed.