As per the following question it appears not to be possible to use the RawInput API to swallow key input: Is it possible to swallow a key in Raw Input?.
One other possibility would be to set focus to my application from the RawInput API and handle the input in the main form's KeyPress event. This way I can suppress the keys by settings the e.Handled state to true.
Another alternative would be to use both a global hook and the RawInput API, but this would require two separate processes as the RawInput API unhooks any global hook within the same process.