In order to disable modifying tag contents after the tag has been marked, the following script should be installed as $SVN_REPO/hooks/pre-commit
#!/usr/bin/perl
my $repo = ${ARGV[0]};
my $tx = ${ARGV[1]};
my $changed = qx{/usr/local/bin/svnlook changed -t $tx $repo};
die «Rejecting commit under tag: \n$changed»
;