How can I customise the 'Lost your password?' error?

I have a site which is to be used by many users all sharing a single username/password. The users will receive the username and password separately.

I notice that if a user mis-types the username or password at the WordPress login screen, an error appears:

ERROR: The password you entered for the username test is incorrect. Lost your password?

The Lost your password? text links to /wp-login.php?action=lostpassword which then allows the user to enter an email address to receive a password reset link.

Because I cannot have any user reset the password (this would affect all users) and because they cannot enter an email address to do so (as they won't know it), I need to either prevent users - or just this user - from resetting their password... or I need to customise the error so I can remove the link and provide them with meaningful instructions on where to find their login details.

I've tried using the Prevent Password Reset plugin from Justin Tadlock but this doesn't achieve anything I need.

To keep this simple - is there a way I can customise the error message seen by the user when they enter an incorrect username/password?