SHOW SLAVE HOSTS shows incorrect output
You will have to check the following items


If the decommissioned Slave server still has /etc/my.cnf, do one of the following

rm -f /etc/my.cnf
mv /etc/my.cnf /etc/my.cnf.old

If the Master Server still has any --report-host in its /etc/my.cnf

remove report-host from /etc/my.cnf
service mysql restart

You may need service mysql restart on the Master anyway
Make sure you delete master.info in the datadir of the old Slave


Give it a Try !!!