I picked up this idea elsewhere and thought it was a good suggestion.
You could get the JW a small gift and thank them for the studies that you have had together. Explain that you've gone as far as you are happy with and don't feel you would get anything out of studying any further.
By giving a gift you allow yourself to feel less guilty and your kindness makes easier when you meet in public.
Not that you should feel guilty anyway. As already mentioned, you should have been dropped as a study long ago as you weren't 'progressing' by their standards and that is what they have been told to do. Also, the JW was doing nothing extraordinary as they've been trained to do this. They record their study time and get to mark it down as a study. They report this to their Elders with pride. It's not just in your interest that they have been doing this in the first place. They also expect that most of their studies will not get baptised eventually.
Currently it takes over 12,000 witnessing hours to get one new recruit and that new recruit may be a family member so it takes many more hours to recruit others.