Do you use ASM, or files on disk? If you use ASM then:
connect target /;
startup mount restrict;
Then go and delete all of the directories with datafiles, then delete all of the dump directories
sqlplus '/as sysdba'
show parameter dump
show parameter diag
select file_name from dba_data_files
select file_name from dba_temp_files
order by 1;
and if this is 11g delete the directories with the sid name under diag/rdbms.
You should make sure you are deleting the right database first.