How to deauthorize devices without a computer?

Presumably the message was along the lines of remove a device (iOS devices aren't authorised/deauthorised) from the 'manage devices' section of your account, there is a limit of 10 devices/computers that can be associated to an account at a time.

If you still have a device that you can remove (assuming that it's been associated to the account for 90+ days) you can tap on your id in Settings > iTunes & App Store on it, tap 'view Apple id' on the popup and log into the account, and you might have a 'Remove This Devices' button under a 'iTunes In The Cloud' heading. To remove an association on a device that you don't have access to you will need to use a computer's iTunes (a family member or friend hasn't got a computer with iTunes on ?) or try contacting iTunes Support and see if they can remove a device for you, use the 'contact us' link at the bottom right of every page on here.

What exactly are you wanting to do ? iOS devices (iPads, iPhones, iPod Touches) aren't authorised/deauthorised, an iTunes account is only authorised/deauthrised on computer's iTunes.