[Q] Rooted S5 can not remove apk file in /system/app

My Samsung Galaxy have been inject some virus in /system/app. Now I root my phone and run rm command by root user, but still can not remove the apk file. The system show:

rm failed for /system/app/providerdown.apk, Operation not permitted

I want to know why the root use don't have the permission to remove some file?

I post the cmd log of my operation:

root@android:/system/app # ll -la |grep providerdown.apk

-rwsr-sr-x root root 178557 2015-06-11 14:07 providerdown.apk

root@android:/system/app # mount -o remount,rw /system

root@android:/system/app # rm -rf /system/app/providerdown.apk

rm failed for /system/app/providerdown.apk, Operation not permitted


Any one can help me and Thank you for your ANSWER!