If you don't want to show a core, or contrib message to the end user, you now have to iterate $_SESSION['messages'] and search for the (translated, token replaced) string to unset it.
If a message has a unique key, you can easily override a message, or unset it.
function backdrop_set_message($message = NULL, $type = 'status', $repeat = TRUE, $message_key = FALSE) {
A simple unset($_SESSION['messages']['status']['unique_key']) could remove the message, and $_SESSION['messages']['status']['unique_key'] = t('Something else'); changes it.
GitHub Issue #:
1062
Recent comments
@willowf If you have the time and patience for OG I don't think you'll regret it.
Create groups or communities with nodes or Organic groups
And another note: OG has a bit of a learning curve. It's not very intuitive, but I believe there is good ducomentation in its wiki page. https://github.com/backdrop-contrib/og/wiki
Create groups or communities with nodes or Organic groups
Organic Group is a very comprehensive (and somewhat heavy) solution (disclosure: I'm a maintainer). If what you need is fairly simple (from your description that seems to be the case), what you...
Create groups or communities with nodes or Organic groups