Not sure about renaming, but you can remove a device and/or path by using the following commands, to remove a controller, see below. Same idea to remove an MD group.
(Demo_ArubaMM_01) [mynode] #show switches
All Switches
------------
IP Address IPv6 Address Name Location Type Model Version Status Configuration State Config Sync Time (sec) Config ID
---------- ------------ ---- -------- ---- ----- ------- ------ ------------------- ---------------------- ---------
192.168.100.54 None Demo_ArubaMM_01 Building1.floor1 master ArubaMM 8.0.1.1_58792 up UPDATE SUCCESSFUL 0 73
192.168.100.53 None Demo_ArubaVMC_01 Building1.floor1 MD ArubaVMC 8.0.1.1_58792 down NO MM LICENSE 10 73
192.168.100.58 None Demo_ArubaVMC_02 Building1.floor1 MD ArubaVMC 8.0.1.1_58792 up UPDATE SUCCESSFUL 30 73
(Demo_ArubaMM_01) [mynode] #cd Demo_ArubaVMC_01
(Demo_ArubaMM_01) [00:50:56:b0:03:9f] #cd mynode
(Demo_ArubaMM_01) [mynode] (config) #no configuration device 00:50:56:b0:03:9f
Warning: Device 00:50:56:b0:03:9f will be reloaded, if present
(Demo_ArubaMM_01) [mynode] #show switches
All Switches
------------
IP Address IPv6 Address Name Location Type Model Version Status Configuration State Config Sync Time (sec) Config ID
---------- ------------ ---- -------- ---- ----- ------- ------ ------------------- ---------------------- ---------
192.168.100.54 None Demo_ArubaMM_01 Building1.floor1 master ArubaMM 8.0.1.1_58792 up UPDATE SUCCESSFUL 0 73
192.168.100.58 None Demo_ArubaVMC_02 Building1.floor1 MD ArubaVMC 8.0.1.1_58792 up UPDATE SUCCESSFUL 30 73