You can supress such messages on firewall (using some “rule” to reject/drop such messages/packets) or syslog level (filter them using regexp or wildcards and log them to file or drop).
https://www.syslog-ng.com/technical-documents/doc/syslog-ng-open-source-edition/3.21/administration-guide/55#TOPIC-1180572
https://openwrt.org/docs/guide-user/firewall/firewall_configuration#rules
I think that firewall approach is better as touching the filter in main syslog-ng can bring some other issues related to nikola.sh and parsing of iptables log file. If that message you want to supress is consistent in some values/pattern rule is very possibly the ideal way.
Aside “dmesg” is just buffer/cache in memory , so if this is just in dmesg output, you do not need to filter those at all, they are flushed on-fly …, if you want to keep the initial dmesg buffer after boot (so you can inspect it and see what is happening before syslog-ng hits in and during bootup, use /etc/rc.local and flush the dmesg buffrer to some file before syslog-ng discard that, resp. dmesg will fill it with new entries… ).