Here's another approach using an environment
. The advantage of this approach is that you can decorate the content of the environment globally in your preamble. You could combine this idea with Werner's to make the border more colourful.
I have used the needspace
package to make sure that the \hrule
doesn't get separated from the last line of the paragraph.
\documentclass{article}
\usepackage{lipsum} % needed for sample text
\usepackage{needspace} % provides the \needspace command
\newenvironment{bottomborder}%
{} % code to be processed before the environment begins
{\needspace{\baselineskip}\hrule} % code to be processed after the environment ends
\begin{document}
\begin{bottomborder}
\lipsum[1-3]
\end{bottomborder}
\end{document}