You mention that your study person has devoted time and years. No doubt you have a bond with them too. Sadly, it is that bond which hampers the ability to say No.
Perhaps you need to write a short honest letter to them and ask that they do not call round again. If that fails then a stronger letter may be needed to either them or the local elders at the hall. I suspect though, that if they ignore your first letter, you will probably feel a' little cross' and that, itself, will strengthen your resolve to tell them No..... But keep it polite, but firm..
As regards disfellowshipping etc, you are not at risk of such treatment...
Wish you well.