why I can't delete some users?, how to "clean VPS"?

hello.

When ISP delivery VPS:

Code:[root@pepsi ~]# cat /etc/passwd | cut -d: -f1 | sort
adm
avahi-autoipd
bin
centos
daemon
dbus
ftp
games
halt
lp
mail
nobody
operator
polkitd
postfix
root
shutdown
sshd
sync
systemd-bus-proxy
systemd-network
tss
whbadmin
whbhelper
[root@pepsi ~]#you can see:

avahi-autoipd
centos
systemd-bus-proxy
tss
whbadmin
whbhelper


six strangers users.

then we run:

Code:killall avahi-autoipd
killall centos
killall systemd-bus-proxy
killall tss
killall whbadmin
killall whbhelper

pkill -u avahi-autoipd
pkill -u centos
pkill -u systemd-bus-proxy
pkill -u tss
pkill -u whbadmin
pkill -u whbhelperbut when we try delete:

Code:[root@pepsi ~]# killall avahi-autoipd
avahi-autoipd: no process found
[root@pepsi ~]# killall centos
centos: no process found
[root@pepsi ~]# killall systemd-bus-proxy
systemd-bus-proxy: no process found
[root@pepsi ~]# killall tss
tss: no process found
[root@pepsi ~]# killall whbadmin
whbadmin: no process found
[root@pepsi ~]# killall whbhelper
whbhelper: no process found
[root@pepsi ~]# pkill -u avahi-autoipd
[root@pepsi ~]# pkill -u centos
[root@pepsi ~]# pkill -u systemd-bus-proxy
[root@pepsi ~]# pkill -u tss
[root@pepsi ~]# pkill -u whbadmin
[root@pepsi ~]# pkill -u whbhelper
[root@pepsi ~]# userdel -r avahi-autoipd
userdel: avahi-autoipd mail spool (/var/spool/mail/avahi-autoipd) not found
[root@pepsi ~]# userdel -r centos
[root@pepsi ~]# userdel -r systemd-bus-proxy
userdel: systemd-bus-proxy mail spool (/var/spool/mail/systemd-bus-proxy) not found
userdel: / not owned by systemd-bus-proxy, not removing
[root@pepsi ~]# userdel -r tss
userdel: tss mail spool (/var/spool/mail/tss) not found
userdel: /dev/null not owned by tss, not removing
[root@pepsi ~]# userdel -r whbadmin
[root@pepsi ~]# userdel -r whbhelper
[root@pepsi ~]#why no is possible delete some users??

this is sufficent to "clean VPS" ??
how I can "restore OS to BEFORE of ISP touch" ??

thanks