The owner decides, of course. A block you, the player, can walk through once in a specific amount of time/or only once in general. Should have IP protection too, to avoid people switching accounts. An incredibly useful block/thing, as it would be very useful when setting up prizes. Could also serve as a link: you touch a flag or something, and afterwards, you can walk through the block for a specific amount of time, in spite of leaving the world (could help with disconnects for people playing your world).