I could not access the link because i do not have metalink (and didnt know the support identifier to register for it).
But I found a solution using sysresv on another forum and tried that, as shown below...

Code:
oracle@server ~> export ORACLE_SID=testdb
oracle@server ~> cd /u01/app/oracle/product/10.2.0/bin/
oracle@server /u01/app/oracle/product/10.2.0/bin> sysresv 

IPC Resources for ORACLE_SID "testdb" :
Shared Memory:
ID              KEY
805306484       0xe9389910
Semaphores:
ID              KEY
167772287       0xf09f4914
Oracle Instance alive for sid "testdb"
oracle@server /u01/app/oracle/product/10.2.0/bin> ipcrm -m 805306484
oracle@server /u01/app/oracle/product/10.2.0/bin> ipcrm -s 167772287
oracle@server /u01/app/oracle/product/10.2.0/bin> sysresv

IPC Resources for ORACLE_SID "testdb" :
Shared Memory
ID              KEY
No shared memory segments used
Semaphores:
ID              KEY
No semaphore resources used
Oracle Instance not alive for sid "testdb"
oracle@server /u01/app/oracle/product/10.2.0/bin> cd
oracle@server ~> ps -ef | grep testdb
  oracle  3326 23776   0 14:12:05 pts/1       0:00 grep testdb
It looks like my instance has been completely deleted now, but how can i avoid doing this again after deleting my instance & database via a script (coz at the moment, even after a shutdown abort and then deleting all the necessary files, the background processes still remain)?