The comedian Larry Miller once quipped "I don't understand couples that break up and get back togetherespecially couples who divorce and remarry. That's like pouring milk on a bowl of cereal, tasting it, and saying, 'This milk is sour. Well, I'll put it back in the refrigeratormaybe it will be okay tomorrow.'"
Like any relationship, today’s employers must be adept at spotting when an employer-employee relationship has soured to the point it can no longer be salvaged.
As the old adage goes, one bad apple can spoil the whole barrel. In today’s workplace, one bad employee can quickly turn a positive working environment into a divisive and negative atmosphere. Bad attitudes then seep into performance and before long that bad apple has adversely affected your bottom line. When such an employee is identified swift action must be taken to address the situation to remedy conduct detrimental to the company and to preserve the corporate culture you have worked so hard to engineer.
Perhaps the most difficult aspect of identifying the sour milk is distinguishing between harmless banter and that which truly adversely affects the culture and, correspondingly, the overall mission of the business. 
The hallmark of harmless banter is just that, it is harmless. Chats at the proverbial water cooler concerning office happenings rarely arise to a level of concern. But what happens when, for instance, an employee openly protests performing a function of the job they were hired to do?  Or maybe it occurs when someone crosses the line in an open meeting not only stating that they intend on violating a company policy but encouraging others attending the meeting to do the same. 
Don’t laugh. It happens. And when it does you need to be ready to address the behavior and quickly. Now, it is seldom advisable to do so in the open forum as this may only lead to an escalation of the issue and, effectively a shouting match between you and the offender. But as soon as others are removed from the situation collect yourself and explain that the type of behavior will not be tolerated and that the employee is expected to conduct themselves in a manner which does not involve conduct detrimental to the company. If it persists, you will need to meet again to determine their continued role in the company.
For most this is enough to resolve the issue. Unfortunately, for some, it may not be. Thereafter if the activity continues you may need to address it on a more permanent level. As Larry Miller said above, "maybe it will be okay tomorrow." Once you know that the bad milk is not getting better with age it is time to throw out the bad milk.