I have found that setting the default to paste content in plain text (rather than in source formatting or with merged formatting) will not work in one case.
You can set your preference in File>Options>Advanced>Cut, copy and paste. In the same location, you can deselect the option to have the "paste options" button appear whenever you paste something. To me, it is really annoying to have that little menu button pop up whenever I paste something. But if you do deselect that, then it does not matter if you have chosen to have data pasted in from other programs paste as plain text. They will by default paste with the original formatting, and you have to manually paste it in plain text.
Does anyone else see this problem?