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!