It sounds simple and it kind of is, if your vendor uses well designed software…
The panel must prevent duplicate respondents. The best way to do this is to base registration on phone numbers and email addresses. If the database doesn't do that, then cheaters can create numerous records and have many chances to get into paid surveys.
Yes, people can have more than one phone number and an unlimited number of emails, but phone numbers cost money. When people register, we allow them to provide as many phone numbers as they have. The more, the better. This way, there is less chance of a second record being created for a person.
If someone happens to register twice using different phone numbers, once we find them we consolidate those records into one family.
BLACKLISTING: Once a respondent is found to be a cheater or disruptor, they are blacklisted, so they can never be used again. Once blacklisted, they will not be sent any survey invitations. If they call in, our recruiters look them up and tell them there are no studies for which they qualify. They are still allowed to log in and update their profile and contact information, never the wiser that they have been outlawed.
QUESTION FOR YOUR RECRUITER: How do you prevent duplicate respondent records? Answer should be: Registration keyed on phone and email address which prevents duplicate entries.