I'm not very clear about your requirement.
If you client use dose not have permission to delete the file ,why do you let the user delete it ?
If you really want to delete it using admin accout,you still have several options :
option 1:
Rewrite your webscript ,and make it runas admin account.
option 2
1.Write a custom proxy servlet.
2.When client user need to delete a file ,he/she submit delete request to your proxy servlet.
3.In you proxy servlet ,reading admin username/password form configuration file ,and use this admin account to delete that file.