In case the link doesn’t work in the future:

Method 1:

Let .<ext> be the extension you want to deassociate.
With a registry editor (e.g. regedit.exe),
1. go to
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\
remove the subtree .<ext>
2. go to
HKEY_CLASSES_ROOT\
remove the subtree .<ext>
remove the subtree <ext>_auto_file

Method 2:

Open elevated command prompt and enter
assoc .<ext> = “”

This site makes me feel stupid.