This behavior is known as click fraud and Google has a few methods in order to hinder this. If a bot is being implemented to continuously click on an ad, Google will block activity from its IP address. You will therefore not have to pay for any clicks from that specific address. However, if the ad is clicked on manually, if will count as a normal click. You can however manually block clicks from certain IP addresses, meaning you can stop competitors using up your valuable budget if you strike first.