By no means a new item but just archived because I was looking for it on the web and spent more time than I wished to get the right answer. The solution from HowToGeek did the trick:
browse to the following key:
HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \ Explorer \FileExts\Here you’ll see a list of all the file extensions that are registered on your computer.
Browse to the file extension you wish to edit, click the white triangle beside it to see the subfolders, and select OpenWithList. In our test, we want to change the programs associated with PDF files, so we select the OpenWithList folder under .pdf.
Notice the names of the programs under the Data column on the right. Right-click the value for the program you don’t want to see in the Open With menu and select Delete.