Make it possible to flag one square of blocks each day, creating a timer after which they can be picked up by anyone.
This suggestion aims solve the problem of the world getting cluttered with random blocks, while creating the least hassle possible for active players but still effective in removing the cluttering.
The most basic and effective method I can think of is to make it possible to "flag" objects for removal of ownership. Maybe you have one flag each day, that you can use on the blocks in one square (containing all blocks in z-axis) . If a square is flagged it's also immediately obvious to everyone. The owner of the blocks have one week (for example, any time can be used) to click the blocks to remove the flag, and if not done in time all blocks are free to be picked up by anyone. Also, blocks inside towns and premium objects should have a longer timer (like once every month, for example).
If this solution is implemented, all you have to do to keep all your blocks is visit them once a week and clicking on the flagged ones. But it is very effective for removing all the cluttering blocks made both by players who have stopped being active and blocks made by active players but that they've forgotten about. (I know I placed some random blocks while new that I can't rememer any more and that might be in the way for someone.) It also creates a lot of incentive for players to use their flag each day since doing so will make a new way to get blocks besides buying them.