Munin tends to spam the log files it writes (munin-update.log, munin-graph.log...) with many lines at INFO log level. It also doesn't respect syslog log levels as it uses Log4perl. In difference to the munin-node configuration (munin-node.conf) there is no "log_level" setting in the munin.conf at least in the versions I'm using right now. So let's fix the code. In /usr/share/munin/munin-<update|graph> find the following lines:
logger_open($config->{'logdir'});
logger_debug() if $config->{debug} or defined($ENV{CGI_DEBUG});
and change it to
logger_open($config->{'logdir'});
logger_debug() if $config->{debug} or defined($ENV{CGI_DEBUG});
logger_level("warn");
As parameter to logger_level() you can provide "debug", "info", "warn", "error" or "fatal" (see manpage "Munin::Master::Logger".
And finally: silent logs!