Completely agree with Debroop and Senthil.
As there is now way to remove the action item from XOG although we can update the state of an action item via XOG.
Refer the below XML:
<NikuDataBus>
<Header action="write" externalSource="NIKU" objectType="actionItemApproval" version="12.0"/>
<actionItemApprovals>
<actionItemApproval actionItemDbId="5038001" assigneeDbId="5038001" processDbId="5084022" status="CAL_CLOSED" />
</actionItemApprovals>
</NikuDataBus>
But if any process is associated with the action item then it will get stuck.
Or we can remove the action item directly by updating the database but that is not recommended by 'CA' and it will also stuck the process.
Note: It will work only for the logged in user.