Here you are experiencing two security features of the engine. The first
one is the multiple level administration (a.k.a. MLA). The engine
organizes objets in hierarchy: a set of data centers, then inside each
data center a set of clusters, and inside each cluster a set of virtual
machines. When you assign a permission to a user or group you in fact
assign it to one of these objects, and objects deeper in the tree
inherit them. So if you assign the "create vm" permission to user A and
the default data center, then that user will have permission to create
VMs on any cluster of that data center. I guess that the two users that
you initially created had the permission on the default data center or
the default cluster, so they have the permissions apply to all the VMs.
Try to go to the data center or cluster tabs and see if the users have
permissions there, then remove them as needed.

The other thing you are experiencing is the prevention of hijacking of
the console of a VM.