Don't know if there is an option to switch off logging, but there are options to specify the paths for the log files. So you could let it write to RAM: (so that it gets deleted every reboot) or NIL:. To see these options you have to switch to advanced mode in SWAT.
However, I am not sure what nmbd logs but I don't remember this file growing so big. Did you look into the log ? Perhaps there are some error messages which point you to a misconfiguration you could fix.
Bye,
Thomas
