smbstatus can tell you what files are being locked but it doesn't directly give you the user who is locking them. I wrote a little bash script that will tell you who is locking files on the samba server. Usage is simply: smblock.sh foo
This will list the users who are locking file 'foo' (or 'foobar' or 'foofyfoo' etc). You don't need to use the full path. Get it here.