For added security (and reduce unwanted spam), you can require users to complete a Google Captcha challenge. This is verify people are who they say they are.
To enable this: navigate to New Giveaway (or edit an existing one) > Basic Information > then look for the header that says “Google Captcha”. Then, select yes.
Here is an example live giveaway that requires Google Captcha to enter.
Additional security measure you can take: