Are these VM's part of one pool in Horizon Admin or multiple pools?
Depending on how you have the pool configured Horizon will try to keep the minimum number of VM's powered on to meet requirements of the pool.
If you don't need/want users logging in you can disable the pool and power off the VM's and they should not power back on.
At a minimum Horizon will keep 1 VM in every pool powered on. You can overcome this by placing the VMs in maintenance mode.
Are you trying to access Horizon Administrator directly on the connection server? It looks like there is a policy on the server preventing internet access. Try accessing it remotely.
Thank you so much to both of you, both posts have helped me resolve this issue. It has led me to the admin page where I was able to see that these VM's were part of a 'desktop pool'
Once I lowered the number to 1 I could then power down the machines. It looks like whoever used to administer these machines put them into maintenance mode as one of you had suggested.
I'm very grateful for both of you as this issue had me tearing my hair out!!!