How to clear an input value on some button press?

Use Event.preventDefault().

Add preventDefault() in your clearInput code as shown below:

clearInput (e: KeyboardEvent) {
   if (e.keyCode === 44) {
      e.preventDefault();     // <-- Here
      this.element.nativeElement.value = '';
   } else {
      console.log('Not a comma');
   }
}

Simply return false if a comma is pressed:

class HomeComponent {
  @ViewChild('elem') public element;
  clearInput(e: KeyboardEvent) {
    if (e.keyCode === 44) {
      this.element.nativeElement.value = '';
      return false;
    } else {
      console.log('Not a comma');
    }
  }
}

JSFiddle: https://jsfiddle.net/lucakiebel/zrehcwfy/1/