Curious Google Adwords Question

I have some of my clients who run Google adwords thinking that their competitors may just be clicking on their ads to use up their credit.
I know that You only get charged once per computer/IP but was just wondering that maybe their competitors could be changing their IP address using a VPN and clicking on the ads
so Google will think it's a new computer.

Is this possible or some way to track it?

google know it's a proxy ip or not, so you will be safe, google make sure you get real, quality traffic, I have spend $30.000< on adwords and I know what in this game.

Ok great, just some peace of mind for my clients.
I've heard people use VPS and VPN to change everything as if they;re on a new machine, I know what a VPN does but not sure what a VPS does, what are your thoughts on this?

Things like this does happen! When Google comes to know about it they usually stop showing that particular ad to the clicker!

I heard that google still admits that around %20 of clicks are fraud...