An answer form someone who is by no means an expert but sometimes has ideas:
First write this function:
FUNCTION A_letter_has_been_typed: BOOLEAN;
VAR
I:Byte;
Begin
Result:=false;
FOR I:=Ord('A') TO Ord('Z') DO
IF Getkeystate(I)<-125 THEN
Begin
Result:=True;
Break;
End;
End;
And then, start your Onclick procedure like this:
PROCEDURE TForm1.Cbb_fontnamesClick(Sender:Tobject);
begin
IF A_letter_has_been_typed then exit;