This is one of the things we're hoping to fix, because I get asked about it A LOT. Here's the boilerplate answer:
Unfortunately, our automatically generated renewal reminders do not recognize a Lifetime membership as a reason not to pester you. This means that you can ignore and delete the total of three reminders you'll be getting. This also means you'll still get all three. :( I'm sorry for the inconvenience, but you truly are set for life. :)
What happens is that the one-year subscription IS expiring, which triggers the automatic reminders. Those go out at 21 days, 7 days, and 1 day from expiration. They they go away.
The only other possibility (which I made sure wasn't Nathan's case) is that users sometimes create entirely new contact records when they order the lifetime subscription. In those instances, I move the lifetime sub to the proper account, which doesn't stop the renewal emails, but makes them safe to ignore again.
Hope this helps!