Need to have same password for multiple respondents using anonymous link

I’m doing a project where we are doing intercept interviews during a nightly event in several units of a hotel chain. We are handing out little cards with a survey URL asking people attending each nights’ event to take a 5-minute survey on their own smartphone during or within 3 hours of the end of happy hour. Anyone who completes the survey is receiving a $5 Amazon gift card code via email. Our concern is that some less-than-honest people will either attempt to take the survey more than once on different devices using different email addresses, and/or will send the survey link to friends and tell them to fill it out and they’ll get a free $5 gift card. I do NOT want to have to give each respondent a unique ID, as that would be a nightmare for printing up the paper cards we are handing out. What I’d like to be able to do is have everyone taking the survey to have to enter a password—with everyone entering the same password. Then 3 hours after nightly event ends each night, I want to be able to easily change that password so the original one no longer works. Is there a way I can use the Qualtrics authenticator to do this, or is there some other alternative I’m not considering? Any thoughts would be appreciated.