Turns down an application to one of your approval-mode listings. listing is one of your open listings on this server. user is a text field, not a member picker: paste an @mention or the applicant's user ID. If Group Finder is switched off for the server the reply is Not enabled.
The listing must be yours: Listing not found among your active listings. Text that is not a mention or an ID gives Provide a @mention or raw user ID. If that person has no waiting application: No pending application found for that user.
On success the reply is Application from @user denied., the applicant gets a DM Your application to **{title}** was not accepted., and the card refreshes. A denied member can apply again later; only members the leader removes from a group are blocked from coming back.
Like /group approve, this is a leader action that sits in the moderation category of the command access presets, so a Default preset limits it to the moderator roles.