How to get unique worker for each HIT? Or set a limit to the worker.

Hello All, I have recently launched a study on AMT where I launched 260 HITs. I have collected three unique responses for each HIT from three distinct workers. That made 260*3= 780 unique datapoint for my study. However, only 35 workers worked on the 260 HITs. I have a separate qualification where i already approved 2000 distinct workers. Now, for my next study I want to collect 260 distinct worker's responses for 260 HITs. How can I do that? There is no option for limiting workers for working on multiple HIT. at the AMT requester interface. Please help me if anyone knows.