You can use a hook (put it into <repository>/hooks and name it pre-commit.bat (Windows)):
@echo off
::
:: Stops commits that have empty log messages.
::
setlocal
rem Subversion sends through the path to the repository and transaction id
set REPOS=%1
set TXN=%2
rem check for an empty log message
svnlook log %REPOS% -t %TXN% | findstr . > nul
if %errorlevel% gtr 0 (goto err) else exit 0
:err
echo. 1>&2
echo Your commit has been blocked because you didn't give any log message 1>&2
echo Please write a log message describing the purpose of your changes and 1>&2
echo then try committing again. -- Thank you 1>&2
exit 1
src: http://www.anujgakhar.com/2008/02/14/how-to-force-comments-on-svn-commit/