It is actually quite easy to turn off forcefield. Click on There, then Customize, and World, and Forcefield. It's a toggle, so you click it to turn it on, and then click it again to turn it off.

When it's off, the little yellow 'halo' around your name tag will disappear. When it's on, the little yellow 'halo' is easily visible around your name tag, showing it's turned on.

You can turn on your forcefield to prevent your avatar from being hit by vehicles or paintguns. We strongly recommend the use of forcefields when you are being 'Griefed' by other avatars who recklessly or maliciously go about hitting any avatar in sight with their paintgun or vehicle.