Step1 - Login as 'oracle' user and stop the listener
$ORACLE_HOME/bin/lsnrctl stop
Step2 - Login as 'root' user and Delete all directories and files files from $ORACLE_HOME
cd $ORACLE_HOME
rm -RF *
Step3 - Delete all directories that are lying under $ORACLE_BASE
cd $ORACLE_BASE
rm -Rf admin doc kre o*
Step4 - Delete /etc/oratab file (if any)
rm /etc/oratab
Step5 - Delete oraInventory directory. (Assuming that you have only one db installation)
Step6 - You can now try reinstalling the 11g (11.2.0.2) DB