When setting the world spawn on a multiplayer server with other players, if the spawn is set within the range of a block that can be right clicked (eg Furnace, Crafting Table, Bed) - it will prevent other players from using those blocks. It does not, however, prevent the player that executed the command from using the blocks.

One of the affected players at the time was an OP, (/op) - and another not.

The players were taken away from the spawn point to test a crafting bench, which worked. When taken back to the spawn point area, they could still not interact with the blocks. Once the spawn point was moved away (500 blocks approx), they could use the blocks again.