The man page explains it quite nicely. If it’s just DHCP you don’t want to run on wlan0 then you can use --no-dhcp-interface=wlan0. If you don’t want dnsmasq to listen at all on wlan0 then you can use --except-interface=wlan0.
If you only want dnsmasq to listen on eth0 then you can use --interface=eth0.