AjaxButton does not respect return false on client side

I have an ajax button that launches a modal form. It binds to:

<input type="button" wicket:id="btnCopy" onclick='return false;' value="Copy
To">

I've replace the javascript call that checks the condition with a flat
return false to simplify things. This still launches the event, and renders
the modal window.

The button code itself:

AjaxButton btnCopy = (AjaxButton) new AjaxButton("btnCopy") {
private static final long serialVersionUID = 1L;
@Override
public void onSubmit(AjaxRequestTarget target, Form<?> arg1) {
onSRTabCopy(target);

@Override
public boolean isEnabled() {
return true;

}.setDefaultFormProcessing(false);

Is wicket doing something to get in front of my return false?