The question title really says it all -- the documentation describes how to remove network cards (i.e. pulling the network adapter out of the VM), but not removing a VM from an existing network.
You may not be able to remove VMs from a network, but you can certainly switch which network they are associated with. (One way you could remove a VM from the network, is, well, remove the network card ;))
Steps are given under vSphere ESXi 4.1