This isn't quite solved yet. While setting ExtendedUIHoverTime high enough keeps them from appearing during a regular rollover, they still appear at times for me.
For example: Drag a task around the taskbar a little bit and then release it. With your mouse still on it, click it and then move your mouse. The thumbnail will appear again.
I want these damned things GONE! Like we had in Vista. Why does MS insist on removing useful options like this? I'll run a modified Explorer.exe if I have to, I just don't know where to look to disable these.