VDI - prevent to start VMs

We have Windows Server 2012 - VDI - pooled collection.

I don't understand how RD Virtualization host agent starts VMs in pool.
I would like to prevent that VMs are started when VDI is not in use.
Normally would be that VMs are in saved state, ready.
Are VMs started also when collection is not visible?

We could stop RD Virtualization host agent, but with this we disable all collections.

Thanks for help in advance!