How to Remove Delete aquota.user aquota.group Operation not permitted

While I try to remove Delete the Disk quota file aquota.user,aquota.group I can not delete that file.Because that quota it is not give the full permission.
So I will try to give the full permission for that particular file.But I got the following error message.
[disk@rhel5 home]# chmod 700 aquota.group
chmod: changing permissions of `aquota.group': Operation not permitted
Then I try lot of method but there is no use.
Finally I turn off the quota in the Home directory
then I try it. The problem will be resolved to delete file.
[disk@rhel5 home]# quotaoff /home/
[disk@rhel5 home]# chmod 700 aquota*
[disk@rhel5 home]# ll
total 14
-rwx------ 1 disk disk 0 May 8 13:20 aquota.user
Then I remove that quota file

ERROR MESSAGE
chmod: changing permissions of `aquota.group': 
These is also applicable for aquota.user
Operation not permitted