If you want to force justification on a single paragraph, you might try
"distributed" alignment (Ctrl+Shift+J).
It's usually used only for Asian text though, and spreads out the letters a
bit.

I sometimes use it to fit text exactly in a given space, and then increase
the font size until it looks nice.

To remove it, just apply some other alignment (left, justified...).

Greetings,
Klaus