This used to be a major problem in Old Habbo because there was almost no way to stop people from blocking. In the New Habbo update, an option in room settings was added to disable blocking in the room, meaning that users can walk through each other, so no one can block.
In 2009, Habbo allowed for the first tile by every door to be unblockable by people. This was the same technology they used for the Lido Pool in late 2008.
Habbo later developed a system to let people choose whether they want blocking allowed or not in their user rooms. Only the owner may set this option in room settings.