Inc.com columnist Alison Green answers questions about workplace and management issues--everything from how to deal with a micromanaging boss to how to talk to someone on your team about body odor.

A reader asks:

I've been receiving complaints from some of my employees about another employee. Let's call her Leah.

Leah is a very good employee. She meets her goals every day and is always happy to lend a hand. She's eager to learn other parts of the job so she can help out.

According to others in her department, though, the reason she's so eager to help is she wants to prove she can do the job all by herself and we can get rid everyone else. Often she sneaks behind people's backs and finishes half-done tasks. These tasks may be left undone for many reasons, and there have been times when she's caused a major snarl by shipping products that aren't fully packaged or something akin. Some people have reported that she goes through and pulls the best products for herself, leaving others with substandard products. She seems to think that she can decide her own work duties, including dictating how her peers operate.

Twice now we've had to go over the harassment policies, and at least one if not two employees have quit over some of her comments (which management only ever hears of secondhand).

I have told her directly to stop doing these things. I'm concerned that it may affect her relationship with other employees, as she's been known to take it out on other people when she gets in a foul mood.

Currently we're in the process of hiring a new full-time position. Leah is very interested in that since she's only part-time, and has upped her behavior to try to push out any other in-house candidates.

How can I approach her behavior? She does her job very well. She just tries to do everyone else's as well.

Green responds:

Whoa. Leah doesn't do her job very well, because part of her job is getting along reasonably well with co-workers, not messing up their work, and stopping behaviors that you've ask her to stop. So the first thing here is to reframe your opinion of her as a "good employee" who's just too enthusiastic and eager to help. She's not.

This is someone who sounds like a horribly toxic influence in your workplace, who has driven off other employees, who takes her bad moods out on other people, who flagrantly ignores the boundaries of her role, and who ignores you when you directly tell her to stop. Frankly, she sounds like someone you should be thinking about firing if this continues.

That means the next step here is much more serious intervention than you've done so far. You need to give her clear, direct instructions about what needs to change and a warning about the seriousness of the situation, and then you need to watch her closely to see if she makes those changes or not.

Start by sitting her down and saying something like this: "We've talked in the past about X, Y, and Z, but the problems have continued. These are serious concerns and they could jeopardize your job here, so I want to be very clear: I need you to stop doing work that hasn't been assigned to you. That's not making you more valuable; it's causing real problems. I also need you to change the way you interact with co-workers. Having pleasant, cooperative relationships with co-workers is as much a part of your job as any work I assign you. That means [specifics of what you need her to stop doing]. Can you do that?"

You should also explain that you can't consider her for the full-time position because these problems are so serious. And really, that does need to be your stance; there's no way you should be considering making her full-time under the current circumstances.

And I'd seriously consider telling her, "If this continues, I'll need to let you go." 

After that conversation, keep a very close eye on her. You mentioned you're concerned that she may respond by taking it out on her co-workers. You want to be watching closely enough that you know about it if she does (don't just rely on someone else to tell you about it) -- and if that happens, you need to immediately address it with her, probably as a final warning.

But the biggest thing here is to change your lens from "good employee who's just too over-eager." That's not the situation you've described. And it's not fair to your other employees if you don't change that framing, and it's not even fair to Leah herself, who apparently thinks her behavior is making her more appealing, rather than less.

Want to submit a question of your own? Send it to alison@askamanager.org.