Campaigns and ads will automatically pause if you run out of funds, or in other words, when the verified balance reaches zero. These campaigns and ads will automatically be restored as soon as you replenish your account, or when your verified balance becomes positive again.
Your ads will run only when there is a positive, verified balance in your account. You can only spend as much money as you put into your account, and no more than that.
The number of users referred to your site or product (based on traffic logs) should be similar to the number of ad clicks that you're charged for. This won't be 100% accurate due to some browsers removing the referral header, but there should be a strong correlation between the two.
Ultimately, it's about trust. It's not in our long term interest to attempt to defraud our partners as a company. We do well when our partners do well, and providing fake numbers will cause our partners to have a lower ROI, making them less likely to stay on our network.