As any seasoned veteran teacher can attest, some students reach a point where they are no longer capable of learning. Specialized compartments in their brains, for whatever reason, fill up and do not accept any new knowledge. In these cases, what can teachers do?
Teachers must be insistent that the student has mastered everything there is to know, and any future learning must come through self study and speaking in front of a mirror. The student may protest, but you can point out that they have stopped listening to the teacher and have now become their own teacher. Be firm, yet kind with the student when you tell them not to return.
This is a particularly big problem in Japan, where you can talk to the student for an hour at a time and they do not retain anything that you have given them. Having spent the past hour reading a list of important words to a student, and having them leave the class with a bewildered expression does not result in good feelings.
For students that are insistent that you continue teaching them, convince them of their expertise by artificially dropping your own English ability. Remove prepositions from your conversation, avoid plurals, and interspace your sentences with hoots and whoops. It truly is a confidence booster for the student.