If they stopped playing, I absolutely agree, this is one of the concrete problems that removing the ability to prepay taxes for a ludicrous length of time would solve. Removing land with prepaid taxes that's owned by banned accounts, accounts that have "cashed out" by selling gold, and similar would also help, wouldn't it?
But if they're logging in to pay taxes, then they haven't really stopped playing, have they. If you're interested in working on their land, you can always mail them and ask them to put it on public or family or guild setting.
You have to realize that trying to solve this with game mechanics won't do anything, since it's a problem with the actual players who are doing this, all game mechanics that you can think of will hurt legitimate players much more than the hypothetical RMT P2W 10 accounts land baron who just does it because he can. Unless you do what I said in my previous post and actively police people for generally being terrible human beings, there is no fair solution to it.