Killing keyboard events

In Delphi 1, I've set up an OnKeyDown handler so that I can intercept
control-key combinations while the focus is in an edit box, and use them
for my own purposes.

My question is, how can I then kill the keyboard event so that the
"illegal" keystroke doesn't pass through to the edit box, thus
generating a beep because it's "illegal"? (Changing the "Key" var
parameter in the handler to a null does not seem to do anything.)

Any help will be appreciated.

- Peter Donnelly