Selecting which application opens a file with given creator/type codes (System 7)
I don't have a System 7 Mac to test this on and my memory is a bit hazy - only partly helped by web searches - but I think you want to go to the Mac OS Easy Open control panel and click Delete Preferences.

If you don't have this control panel or something similar then you could also try deleting any likely-looking preference files in the Preferences folder in your System Folder, then restarting; or deleting the hidden Desktop DB and Desktop DF files from the root of the disk, then restarting if it's the startup disk, or ejecting and reinserting it if not. 

Note that all these methods (if successful) will delete all application associations for unknown files, and deleting the desktop files will delete or reset a load of other stuff such as window view settings and possibly Finder comments.