In man portage I see examples of using package.use

Its listed with address /etc/portage/package.use and shows usage like
this:

some/package:useflag   (to add a specific use flag)

I want to subtract a use flag so trying:

 (tried with and without quotes just in case the dash (-) was not
  acceptable unquoted:

net-analyzer/tcpdump:"-samba"
net-analyzer/tcpdump:-samba

Neither way appears to have an effect on
emerge -vuDNp tcpdump

It still comes up with positive samba use flag. And then when actual
emerging is done, dire warnings are given about using `samba' use
flag.

I'd hoped to use package.use so as not to have to mess with tcpcump
individualy.

Is my syntax wrong or some other problem?

--
gentoo-user@(protected)