I tried using the following veto files parameter in smb.conf to keep a Ubuntu Server samba share clean of OS X files.

veto files=/._*/.DS_Store/

Everything seemed to be working great until a user on Snow Leopard attempted to copy and overwrite an existing file on the share and received the following error:

The Finder can’t complete the operation because some data in “index.html” can’t be read or written. (Error code -36)”

Oddly, I was unable to reproduce the problem on my Lion iMac. My initial guesses were Samba’s permission overrides (create mask, force create mode, directory mask, force directory mode, and force group) and the unix extensions parameter, but disabling these didn’t make any difference. After trying everything else, I discovered that Snow Leopard (and most likely older OS X versions) required a ._{filename} to be created in order to overwrite an existing file. Reducing my veto files parameter to the following resolved all of the Snow Leopard issues.

veto files=/.DS_Store/