StatelessAjaxPreventSubmitBehavior を作成しましたが、`AjaxCallListener` がStatefull のためStatelessにはならなかった。

StatelessAjaxPreventSubmitBehavior

import org.apache.wicket.Component;
import org.apache.wicket.ajax.AjaxRequestTarget;
import org.apache.wicket.ajax.attributes.AjaxCallListener;
import org.apache.wicket.ajax.attributes.AjaxRequestAttributes;
import org.apache.wicket.markup.html.form.TextField;
import org.apache.wicket.request.mapper.parameter.PageParameters;
import org.wicketstuff.stateless.behaviors.StatelessAjaxEventBehavior;

/**
 * StatelessAjaxPreventSubmitBehavior
 */
public class StatelessAjaxPreventSubmitBehavior extends StatelessAjaxEventBehavior {

    /**
     * Construct..
     */
    public StatelessAjaxPreventSubmitBehavior() {
        super("keydown");
    }

    protected void updateAjaxAttributes(AjaxRequestAttributes attributes) {
        super.updateAjaxAttributes(attributes);
        Component component = this.getComponent();
        if (!(component instanceof TextField)) {
            attributes.setChildSelector("input");
        }

        AjaxCallListener listener = new AjaxCallListener();
        listener.onPrecondition("if (Wicket.Event.keyCode(attrs.event) === 13) {attrs.event.preventDefault();} return false;");
        attributes.getAjaxCallListeners().add(listener);
    }

    @Override
    protected PageParameters getPageParameters() {
        return null;
    }

    @Override
    protected void onEvent(AjaxRequestTarget ajaxRequestTarget) {
        ajaxRequestTarget.appendJavaScript("alert('test');");
    }
}

何か別の方法があるのかもしれません。