hi srikanth,
this may be a tricky one wrt to active/completed/cancelled type of tasks..
go to NWA > Operation Management > Processes and Tasks > Manage Process,
filter it by 'Started by/Created by', choose that process and click on 'Show Related Tasks', check the latest date/created user there and compare with the UWL, if that matches, then you can delete the required tasks.
for more details go through this link: