I don't use snmpd, so I cannot give you the direct solution.
The part of my syslog.conf which logs to /var/log/messages looks like this:
Check your syslog.conf and try to set different priorities, for example delete *.=info;*.=notice; and set cron,daemon to warn instead of none.
Settings in syslog.conf will affect the whole logging system. It means, that if you set your minimal logging priority to warning
you will disable info
messages from all programs. Consider redirecting lower priority messages to another log file, to keep log/messages clean and yet retain all information.
If you want to change only snmpd logging you must find appropriate option in its config.