Susan Heathfield of about.com says, “How you address negativity depends on whether you control it and how it started in the first place. The timeliness of your intervention also has an impact.”

She offers this advice to deal with negativity from an individual employee.

• Inform the employee about the negative impact her negativity is having on co-workers and the department. Use specific examples that describe behaviors the employee can do something about.
• Avoid becoming defensive. Don’t take the employee’s negative words or attitude personally.
• Focus on creating solutions. Don’t focus on everything that is wrong and negative; focus instead on creating options for positive morale. If the person is unwilling to hold this discussion, and you feel you have fairly heard her out, end the discussion.
• Focus on the positive aspects and contributions the individual brings to the work setting, not the negativity. Help the employee build her self-image and capacity to contribute.
• Compliment the individual any time you hear a positive statement or contribution rather than negativity from her.

If none of the above is working and the employee’s negativity is impacting productivity, workplace harmony, and department members’ attitudes and morale, deal with the negativity as you would any other performance issue.

http://humanresources.about.com/cs/conflictres/a/negativitycures.htm