To exclude participants from participating again, all you have to do is go to your Dashboard and to the Prescreening tab, scroll down to "Previous Studies" and select the study whose participants cannot participate again in the new study. Just move the old study from the left box to the right box.
The previous studies you want to block must be in the "Completed" section of your dashboard. These studies are have been stopped and have all participants reviewed.
If you have multiple old studies, you can move all of them to right box at once. Now click on "Apply previous studies requirement to study" and you're done.
Please note that Previous Studies will also exclude participants who have made Returned submissions in your previous study.