Someone who isn't a moderator, but attempts to enforce rules without the power of the platform. An example of a problem user is one who tells members how to post, or that they should not post, as though they were an official moderator. The person doing this is inherently abusive or dismissive and not constructive to community building. They may mean well but they often don't understand the community in the first place, so moderation for them is a power-trip, and not something they are effective at. Alternately, they may try to drive the community to move in a way that was not the original intent of the community, thus making it more hostile to new members than how things began.
On reddit, you can remove comments or posts, as well as mark users as spam (so the system learns to reject their posts), but you can also indicate the seat of privilege by marking the post as from an authoritative user in the subreddit. On Stack Exchange there are many tools for moderation, but moderators are denoted by a diamond next to their name.
What is the best approach to reigning this member in, so they are more active but not "abusive" to the community?