The written word, either via email or IM, is easily mis-understood. However, there are certain words and phrases that are clear and leave no room for interpretation consequently employees shouldn't be required to tolerate them. It's up to the business owner to deal with these people. Otherwise, lacking those certain words and phrases, personnel should take a deep breath and choose to interpret them as more positive than negative.

Previous posters have made some very good points. However, it seems that part of the problem may be customers that are trying to negotiate a better deal. I'd classify these as aggressive (read: aggressive negotiators, versus abusive) customers. Again, it's up to the business owner to deal with this type of customer, unless the employee is empowered to make deals beyond that which is advertised.

That said, business owners have the right to deny service to anyone as long as it's not based on race, creed, religion etc.