Three choices come to mind; use longer or shorter words or don't justify.
Opps, a fourth, change font size or style.  Hmmm... that's five.
And! For a sixth, make the work area width to wider or narrower. Egad! that
makes seven.

Here's an eighth but Sarah will scold me, misspell the words. Ouch!

Are you using shift+enter? Turn on special characters, is there a tab?

I had that problem only when I pasted some text into a column that I copied
from a website.  One word would stretch across the column--column width had
nothing to do with it--was in the translation of the sentence end code. I hit
a double delete at the end of each sentence and it worked the way the
"justify" is supposed to work.

You'll probably either have solved the problem or given up by now, but I
thought I'd have a go at answering it, in case anyone is looking for a
solution. I was having the very same problem in a publication I was putting
together with entries from various sources. I was able to justify most of
them correctly in Publisher with no problem, but I had one that persisted in
spreading one or two words across the whole line. I almost gave up, after
trying various cures without success. As a last resort, I copied the text
onto Notepad. I could then see that, between some of the words there were
little black rectangles (don't know the correct name for these characters)
that hadn't previously shown up. I deleted all these, then copied the text
back into Publisher. Hey presto! It justified perfectly!