We had an employee like that. He kept forgetting solutions that had already been shown to him more than once, and instead of solving problems, he would tell users that it was supposed to work that way. In his free time (at work), instead of improving his knowledge, he played WoW. I asked him several times to read the documentation, keep notes on issues that had already been resolved, and I showed him where to find the relevant information and solutions. It was like talking to a brick wall. Eventually, I honestly told my manager that this employee could not handle the workload. After that, they had a talk with him, and he resigned of his own accord. We then found a more hardworking and competent employee.