When you are resigning from a job - it is best to give a brief non-explanation or none at all regarding your decision. Professionalism rules.
When you are leaving a religion... it may be a little different. I would suggest making sure it includes non-refutable arguments regarding their doctrine; emphasizing that "waiting on Jehovah" isn't good enough anymore. Try couching it in spiritual terms, they may leave you alone. Being considered an "apostate" is actually a great protection from harassment and the emotional torture. The shunning begins almost immediately, and while painful, is much better than repeated phone calls, JC's, and shepherding calls.
Of course, they have the imperative to save you. They are going to try - what you have to do is make it spiritually dangerous for them to visit with you. Maybe you can try convincing them to leave WITH you. Go on the offensive for a bit. It will drive them into defensive mode, and then they will never talk to you again.
Avoid becoming overly emotional. I can tell you hate the meetings. The memory of countless wasted evenings leads to my imagination being hauled into the bathroom for a spanking; if that analogy makes any sense. Trauma through boredom. But it is actually irrelevant. They won't leave you alone because you say you're bored. Good luck...