Following custom firewall rule can be added in [Network] -> [Firewall] -> [Custom Rules]:

iptables -A input_wan_rule -p udp --dport 27960 -m hashlimit --hashlimit-mode srcip --hashlimit-above 768/second --hashlimit-burst 128 --hashlimit-name q3server1 -j DROP

Rate (--hashlimit-above) should be big enough to handle at least sv_maxclientsPerIP * cl_maxPackets * (cl_packetdup + 1) packets

You may need to use other than input_wan_rule chains depending from your routing configuration

LEDE/OpenWrt firmware MUST be built with following options:

CONFIG_PACKAGE_iptables-mod-hashlimit=y

CONFIG_PACKAGE_iptables-mod-u32=y (not required, but may be useful to inspect packet's contents)