Remove a File association

C:\Windows\system32>assoc /?
Displays or modifies file extension associations

ASSOC [.ext[=[fileType]]]

.ext Specifies the file extension to associate the file type with
fileType Specifies the file type to associate with the file extension

Type ASSOC without parameters to display the current file associations.
If ASSOC is invoked with just a file extension, it displays the current
file association for that file extension. Specify nothing for the file
type and the command will delete the association for the file extension.

_______________________ EXAMPLE _______________________

C:\Windows\system32>assoc .bin //shows the association for .bin extension
.bin=testapp.exe

C:\Windows\system32>assoc .bin= //removes the association for .bin extension

C:\Windows\system32>assoc .bin // shows association for .bin extension
File association not found for extension .bin

C:\Windows\system32>