Why don't you insert a variable in to the password fields and define the data type accordingly. This will restrict the respondent going into your survey only once.
If you do not know the passwords up front, given they are generated on the fly, then go to the "Settings" tab and click on the "Allow respondents to define their own passwords" check box. Also enter a minimum number of characters for the passwords which the panel should be able to inform you of.
This will allow the panel to allocate passwords to respondents on the fly, without you knowing what they are until respondents click on their respective survey links.
If you need to cater for any additional variables being passed in to your survey by the panel, define these under the pass-in fields.
And a Happy New Year to you too.