@TripleB As far as I know, there is no way to remove that. That's a system generated dialog and I don't think there are any specific preferences to control that, though I can't be 100% sure about that.
What exactly is the need here? Are you just concerned about users saving their credentials into the keychain, or is it because the credentials change frequently and it causes some issues later on? Or something else?
Are we talking file shares or some other server access protocol?
If you use Kerberos based network file shares then you shouldn’t need to submit login credentials at all, which would prevent involving keychain.
Maintaining a valid Kerberos ticket can be done in a variety of ways, but this could be something to consider.