You are able to cancel any recruitment order that has not yet completed. When you cancel a recruitment order, you'll receive a refund of any unused credits from the study.
How to cancel an order
Go to the main Dashboard for Ballpark
Next to your name on the bottom-left corner > Press the three dots (…)
Find the order you would like to cancel on the table
Click on "View details"
Here you will see a breakdown of how many credits are still unused
Press "Cancel order"
Your order will now be cancelled and you will receive a refund of the unused credits, you can see the details of it in the "Credits" tab.