After a worker submits their HIT, you have the option to accept or reject the work. There is no way to avoid paying the worker without also rejecting their submission.
Rejections on Mechanical Turk harm a worker's reputation. Over time a worker who accrues many rejections may find there are few HITs they qualify for or in severe cases the worker may have their account suspended. Because rejecting a worker's HIT has a negative impact, researchers sometimes ask if there is a way to avoid paying workers without rejecting them. Unfortunately, the answer is no.
If you are unable to reject workers based on IRB concerns, we have created a way to keep certain workers from taking your future studies. You can add workers to your personal Universal Exclude list. Workers placed on your Universal Exclude list will be ineligible for all of future studies when you choose to apply the list.