One way to work around this is when you map the network drive on the NT worksystem, specify the Unix user name in the "Connect as" text box. This will require the users to specify their Unix password each time they have to reconnect to the network.
Another way you can try: If you are using the Samba Web admin tool to configure your smb.conf file, go to the Globals page and click Advanced View to get all options that are configurable. In the Security section, there is a Username map option. Specify a username map and save your changes.
In the username map, you can map Unix usernames to the NT logins. I think the format is <Unix username> = <NT username>. Check the help file for that option.
If you aren't using the Web admin tool, the map file is specified in the Global parameters area of the smb.conf file: username map = /directory/filename.
I've never used the username map but I have used the "Connect As" method and it works.