svn merge -c -R repository
where -R is the revision number you want to revert to.
What happens is that if R is negative it will be regarded as an inverse merge and the commit will be removed instead of added.
svn -c -99 http://svn.somewhere.com/trunk
will revert you trunk to revision 99
P.S Don't forget to commit after the merge if you want the reversal to be permanent