Isn't this the use case for a nonpersistent disk? Whenever the VM is rebooted 'hard' via Power -> Reset (you can schedule this within vCenter) the changes on the disk are discarded and the disk goes back into a base form. You can still use OS restart commands to reboot.
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=150
