This is how I fixed my OMV
Modify the snmpd file from the following location
vi /etc/default/snmpd
change this line:
SNMPDOPTS="-Lsd -Lf /dev/null -u snmp -I -smux -p /var/run/snmpd.pid"
to this:
SNMPDOPTS="-LS4d -Lf /dev/null -u snmp -I -smux -p /var/run/snmpd.pid"
LS4d will change the logging to warning and eliminate the snmpd messages from getting logged and going to your syslog server.