This command will stop the messenger service on your pc, this will also avoid that you receive any messages:
c:\>net stop messenger
If you want to stop the messenger service remotely, check out this VBS script with WMI functionality: http://rattlesnake.at.box.sk/newsread.php?newsid=116
If you know how to use VBS, it should be no problem to modify it appropriately if you want to disable a service on different remote computers on your LAN. The service name of the messenger service is "MESSENGER". I am not sure if this will work with computers not on your workgroup.
To block netbios, you should get a firewall like Zone Alarm Pro since I would not advise you to go for a portblocker. However, the netbios service is used for many functions on your network, not only for the messenger service. So I am not sure if this would actually be a good idea.