Not letting a user save their password is user hostile, which is why this is disabled in Chrome.
The question here is - do you want to disable it because you are looking for "added security" ("the user must type their password every time") or because it is not really a password/it is an old password field and this looks like a browser bug because it makes no sense to save that password?
For the former, you will have to use some JavaScript/HTML/CSS hacks in order to make the browser do this (it is not easy).
For the latter, you should file an issue at
crbug.com in order to improve the heuristics of Chrome for saving passwords.
Anyway, autocomplete="newpassword" is actually instructing the browser to offer to save the password. If anything, autocomplete="oldpassword" (I forget whether this is the exact name, sorry, consult the specification) should instruct the browser not to show it. However, if the browser successfully (but sometimes falsely) detects that it is not really an old password field, it will still offer to save the password.