I've created a Windows CE 5.0 application which runs on a handheld scanner. The scanner has its own (hardware) keypad and almost all input comes from the scanning unit.
Unfortunately whenever the text box receiving the scanned characters is focused the input panel appears at the bottom of the screen, blocking almost a third of the screen space.
Is it possible to deactivate it in my form or in the whole application?
That last time I worked with CE was back when it was called pocketpc 2002 (I still have my old iPaq 3870 - one of the first devices with bluetooth and one of the last without wifi), but at that time the simplest way around this was to set the device to use a hand-writing recognition mode that didn't pop up anything. That may or may not be an option for you and things may have improved since then.
If you're not doing it manually via the InputPanel control, then I assume you have aygshell in the image and they are being rendered with a WC_SIPPREF control. I'm not certain if you can remove that control manually - I've never tried. There may be an agshell function that will allow you to disable/remove it, or maybe some work with the InputPanel for your app can remove it.
You may also want to see this blog entry for a bit more detail.