Remove an MSTP instance on a 6224 switch

Hi,

I have 8 switch stacks connected together all with MSTP enabled. We were going to implement MSTP but have decided to stick to just one instance and therefore don't need it. Two of the stacks have MSTP instances (1 and 2) configured but with no VLANs assigned to them. However I'm seeing the following in the error logs on one of them

19351234	Notice	JUL 18 14:03:47	TRAPMGR	0/3/3 is transitioned from the Forwarding state to the Blocking state in instance 2
19351235	Notice	JUL 18 14:03:49	TRAPMGR	0/3/3 is transitioned from the Forwarding state to the Blocking state in instance 2
19351236	Notice	JUL 18 14:03:51	TRAPMGR	0/3/3 is transitioned from the Forwarding state to the Blocking state in instance 2

This is happening constantly every 2 seconds. LAG 3 settings are below

interface port-channel 3
description 'sw131-sw122'
spanning-tree cost 200000
spanning-tree port-priority 128
spanning-tree mst 0 external-cost 200000
switchport mode trunk
switchport trunk allowed vlan add 2,4-5,8-9,58-59,99,103,107-112,114-120

So MST instance 2 isn't configured for that LAG (or in fact any LAG or VLAN).

Ideally I'd like to remove the MST 1 and 2 instances completely but I cannot find a way to do this through the GUI or the CLI

Alternatively would it work  if I change this switch from MSTP to RSTP and would it cause a topology change or a block while it recalculates it ?

Any help or pointers appreciated,

regards,

Paul