What's wrong with Canada and mexico?
You could create a filled map and put borders around the US State shapes (viz the Color shelf). Then turn off all layers on the underlying map. Or find a WMS that only has USA rendered.
I think they are your only options - slightly hacky, I guess.
There's one other option: Create the map you want in a image editing program and then use it as a background image. But getting the lat/longs to line up will also be a bit hacky. You'll need some sort of registration marks in the four corners. Andy's way is a lot easier.
Hey Andy, do you have any good sources for WMS layers? I've been trying to use them in Tableau but am having a lot of trouble. I've never dealt with WMS before.