[nova] allow only specific instances on host aggregate

Hi, I'm playing around with host aggregates and the aggregate_instance_extra_specs for flavors. I successfully created a host aggregate, it allows me to schedule instances with a specific flavor on that aggregate only. But is there a way to prevent other VMs from starting on that aggregate (which were launched with different flavors)? The only thing coming to mind is to put that compute node(s) into a different availbility zone. Since the default is "nova", this would prevent anything else to start on that hypervisor. Is there anything I missed with aggregates? Any pointers are appreciated! Thanks! Eugen