You can use <your repos directory>/hooks/pre-commit
and use some batch scripting (or even a full blown program, as long as it's executable it will be fine). If it returns 0 the commit will be successful; otherwise it will fail.
See post-lock.tmpl
in that same directory for an example.
# PRE-COMMIT HOOK
#
# The pre-commit hook is invoked before a Subversion txn is
# committed. Subversion runs this hook by invoking a program
# (script, executable, binary, etc.) named 'pre-commit' (for which
# this file is a template), with the following ordered arguments:
#
# [1] REPOS-PATH (the path to this repository)
# [2] TXN-NAME (the name of the txn about to be committed)
#
# The default working directory for the invocation is undefined, so
# the program should set one explicitly if it cares.
#
# If the hook program exits with success, the txn is committed; but
# if it exits with failure (non-zero), the txn is aborted, no commit
# takes place, and STDERR is returned to the client. The hook
# program can use the 'svnlook' utility to help it examine the txn.
#
# On a Unix system, the normal procedure is to have 'pre-commit'
# invoke other programs to do the real work, though it may do the
# work itself too.