I appreciate that a number of suggestions have included the thought about being polite. I wholeheartedly agree. But remember that being polite does not mean weakness or uncertainly.
"No thank you" is still NO, it's just being nice about it!
Let your yes mean yes and your no mean no. There's no maybe, possibly or "we'll see" about it.
It's all about setting firm, non-negotiable boundaries.
The problem is that JW culture let's elders cross all sorts of boundaries that they never should cross. And due to the cult indoctrination most of us willingly let them. Once you try to reclaim what's yours it confuses them. But as long as they don't have any evidence of any judicially actionable offense all they can do is stew in their own juices of frustration.
That is THEIR problem. You handle yours, let them handle theirs!