I've had a couple of students who have done literally no practice over months. There have only been a couple, but man it can be frustrating. The worst part is that they don't care that they haven't done any practice, or that they're wasting your time.
I had one student who happily stated she didn't practice, and the reason was 'i dunno'. I had her for 18 lessons, and in the first lesson I showed her how to learn a couple of songs. By the end of the first lesson she could play them HT, albeit a little hesitantly - but you could tell she knew enough to practice at home.
EACH WEEK, she would come back and the notes would be forgotten because she hadn't touched the piano and she wasn't apologetic about it either. I spent the next 17 weeks going over and teaching her the same song, hoping that one day she would come back and play them for me, so we could go onto other material. Never happened. I never taught her anything else apart from those 2 initial songs, and although it was boring on my end (I don't care about her end, considering the half-arsed attitude to piano), I had HOPED that she would have just spent a little time showing me she wanted to learn the piano.
I had another student years ago - same attitude. Then one day she walks in and says 'Since we're moving to california in 3 months, our piano has been packed away in a shipping container, so I don't have a piano to practice on'
Theory... problem solved. What else could I do?
My MAIN point is that despite what their attitude is, I teach them the piano. I don't make it a mission to be evil or boring when they don't practice, but despite the fact that it may be mind numbing, I do what is necessary. At all times, I put in 100% to my teaching, and if the student doesn't contribute anything on their end, then I can hold my head up high and know that I still did my job properly.