To prevent ufw logs from being place into the kernel/syslog, modify /etc/rsyslog.d/20-ufw.conf to include the & stop that's already commented out at the bottom of the file.
# Log kernel generated UFW log messages to file
:msg,contains,"[UFW " /var/log/ufw.log
# Uncomment the following to stop logging anything that matches the last rule.
# Doing this will stop logging kernel generated UFW log messages to the file
# normally containing kern.* messages (eg, /var/log/kern.log)
& stop