Hi Kailas, you can do this in the CLI pretty easily, just make sure you do it to a vserver that you no longer need as all settings will be destroyed and delete!!
For example vserver name = myvserver
::> volume offline -vserver myvserver -volume myvserver_rootvol
::> volume delete -vserver myvserver -volume myvserver_rootvol
::> vserver delete -vserver myvserver
You can then go back into the gui and refresh your svm list and it should be gone.