Permissions on datastores - create snapshot but not new VM

Is there a way to create a role such that adminstrators can create snapshots on existing datastores but not provision new VMs on that datastores?

We'bring up new storage and want to ensure that all new VMs get created over there, but not lock out the admins from doing work on the old datastores.  Is this possible?

It was my understanding that the permission "Datastore/Allocate space" is required to create a snapshot.

We're running vCenter 5.1 with 5.0 on the hosts.

Thanks!