Metal FoX - Invitations for clan merges and alliances have historically been posted on a clan's recruitment thread. As long as the post is polite, which this one seems to be, and they accept a declining of the invitation, and don't repost to push the issue, it's fine. And your deputy's response was a polite "No thank you.", which is what we tell clans to post if they get an unwanted invitation.
Your deputy handled it very well and unless the other clan returns to your thread, there really is no need for any moderator intervention here.