I usually prefer not to advise but I will try stepping into those shoes a moment to say what I might try to do in those circumstances.

In the absence of being able to cut off contact I think I would work on my own acceptance that this person would never change. That it did not make what they were saying true. That the truth was unchanged by their denial.

If appropriate I may try to understand the factors which possibly contributed to them behaving as they have done.

Granted this would not be easy to do if it was someone close to me who I saw often.