How can you get a client to automatically get disconnected from a VPN?

I have a client on a VPN connection who is using all their bandwidth on the VPN, by leaving it on at all times.

Is there any way to have a VPN automatically disconnect after a few minutes of inactivity (Or a few minutes if the inactivity isn't possible).

The client uses the Windows VPN Client, and runs Windows Vista Home Premium, but can use XP Professional if it is needed.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).