<aura:component implements="flexipage:availableForAllPageTypes,forceCommunity:availableForAllPageTypes" access="global" >

    <aura:attribute name="confirmEmail" type="String" />

    <ui:inputEmail aura:id="confirm_email" label="confirm_email" class="slds-input" value="{!v.confirmEmail}"  paste="{!c.stopPasteEvent}"/>

</aura:component>

({
    stopPasteEvent : function(component, event, helper) {

        alert('** fired paste event ');
        var selectedFieldName =event.getSource().get("v.label");
        var selectedFieldValue = component.find(selectedFieldName).get("v.value");
        component.find(selectedFieldName).set('v.value', null);

    }
})

component.set("v.confirmEmail", null);