Since the client is downloadable from the web interface to the ESXi host you aren't going to have much luck with preventing anything unless you repackage every time there is an update. Run the client installer with VMware-viclient /? It will give some options. You could extract and rebuild the install package using the tool that vmware uses (from the /? screen). Make the install silent. put the client back on your ESX(i) hosts. Big job to make that work and keep working.
You could perhaps set a Windows Group Policy disallowing executing the host update utility.
-- David -- VMware Communities Moderator
