I've had to address this in every environment I have supported and it's always a mine field. As end users have learned about snapshots, they naturally want to use them as protections against changes to their production systems. The problem is that once you open this door and word spreads, you will get flooded with these requests. Eventually people are wanting them before every minior patch cycle. For larger environments, with thousands of VMs, this isn't suppotable. I've have implemented varying global policies and red tape to try to stem the tide but it is always difficult to defend against individually requests.
I'm curious how others have addressed this. Please share your snapahot policies and how you protect against this slippery slope.