For example, this should block both the "a" (30) character and the "space" character (57)
_disp = findDisplay 46 createDisplay "RscDisplayEmpty";
_ctrl = _disp ctrlCreate ["RscEdit", -1];
_ctrl ctrlAddEventHandler ["KeyDown", {systemChat str (_this#1 in [57,30]); _this#1 in [57,30]}]