Most networks will provide a "burn pixel" or goal setting. The burn pixel is code you place on a page where, if reached, cancels your ads to that visitor.
Same thing with the goal settings.
Basically, you want to make sure that, if someone buys the product and reaches your thank you or success page, you're no longer showing them ads to buy that product.
There are thousands of complaints from customers of products whose owners are still retargeting them for months after they have already bought. It aggravates the customer and ruins their customer experience.