A click threshold of one would basically mean that every click done by any one person (be it genuine or fraudulent) would have that person's IP blocked immediately and added to the exclusion lists of all of your campaigns.
This is not a recommended setting because of two reasons:
First off, you will be blocking genuine traffic for sure after their first click.
Secondly, the cap placed on the exclusion lists by Google has to be taken into consideration.
The maximum amount of IP's that can be blocked at any one time is 500. Once that cap is reached, our system will start removing old IP's and putting in new ones.
We do have an algorithm in place that prioritizes known bad IP's over potential good ones but this is still not recommended.