Specification of the "-valid = False" parameter (as documented in the smb.conf man page) no longer functions. Any valid user can still connect to the share.
When this parameter is inserted into a share stanza, executing "testparm -s" removes this parameter.
I will fix this today.
Looks like this is also broken in 3.0.x as well. Might have been this way for a while.
This one is fixed with 4f2de297 and friends in all trees.
Closing out bug report.
Please re-open if the patch does not fix the issue for you.