One easy way would be to open the console through an SSH session and paste the line
database put AMPUSER 2014/novmpw no
into your Asterisk CLI (replace 2014 with your extension number).
If you have a bunch of them to do, create a text file and set up the extension numbers one per line. Copy and paste the file at the prompt and away you go.
Note that the following may be more correct:
database del AMPUSER 2014/novmpw
I've never played with it, so I don't know if the novmpw setting being deleted is the preferred method, or if setting it to "no" is the right way. It's pretty simple to experiment with, so you could try both and let us know which worked better for you.