fill out the last bar?

Check http://musescore.org/en/handbook/layout-and-formatting, esp. Last system fill threshold

Changing the last system fill threshold is great for filling all the way to the right side of the page, but if it's only a single measure, chances are it ill look pretty bad if you actually try this. Hard to say in the posted example since there are no notes, but try putting four quarter notes in there and you'll see that stretching it out to fill the width of the page is a very bad idea. You should either change where you put your line breaks so the last line isn't just the one measure, or append a horizontal frame and stretch it out to cover how much right margin you want to leave.

If on the other hand this is meant to just be blank manuscript paper, then indeed, setting the fill threshold to 1% will work fine.