It seems sites did not deleted properly from server you will have delete it manually by editing httpd configuration file and other files. You can use following command to locate instances of that user in your server.
grep example /var/named* /etc/user* /etc/httpd/conf/httpd.conf*
Operating system Ubuntu Linux 10.04.4
ISPConfig Version: 188.8.131.52
Webmin version 1.580
Kernel and CPU Linux 2.6.32-40-server on x86_64
Processor information AMD Phenom(tm) II X4 945 Processor, 4 cores
Real memory 4.0 GB total