It can be difficult when dealing with an employee upon whom you depend because they are competent, but on the other hand are not shored up in the area of manners and the etiquette of your office environment. My approach would be two prong, first of all, I would be very vigilant about finding a possible replacement for this particular employee, in case you need to eventually make that unfortunate move, and at the same time, I would begin an Professional Improvement Plan with said employee to help make them keenly aware of my concerns and expectations as an employee of our company. Starting an improvement plan with the employee will express the seriousness of your concern while also giving the employee specific areas to work on and a way for you to observe and measure if there is indeed improvement.
You can download a Professional Improvement Plan online. Hope this helps....All the best.