Don’t know if this is the recommended way to delete a user, but it seems to work.
sudo service snmpd stop
Open up the snmpd.conf file in /var/lib and find the line with the SNMP user and delete the line
sudo vi /var/lib/snmp/snmpd.conf
Save, exit, and start snmpd
sudo service snmpd start
These steps work for Ubuntu, but should work for any Debain based distro as well as CentOS, Fedora, RedHat etc.