Been banging my head against this all morning.
Basically, I have a listbox, and I want to keep people from changing the selection during a long running process, but allow them to still scroll.
Solution:
All the answers were good, I went with swallowing mouse events since that was the most straight forward. I wired PreviewMouseDown and PreviewMouseUp to a single event, which checked my backgroundWorker.IsBusy, and if it was set the IsHandled property on the event args to true.