Hi Dhshukla,   I have done this many times to get rid of SNMP monitoring alerts on old unused AP that joined the WLC previously and currently in Not joined.   I hope this will work for Devang also.   Best Wishes,

  Remove unused AP from Monitor  > Statistics  > AP Join  > AP Join Stats    Click on small Blue box withe Arrow facing bootom" at the right corner of the AP related Row.  Please find attached screenshots.          

http://7200emu.hacki.at/viewtopic.php?p=34550&sid=400d9ebdbbb4e7eceedc345903dd0faaTry formatting the disk0 partition and reloading:format disk0:reloadThat worked for me