Most servers have a jail around the original spawn to cut down on griefing. I joined a server like this and found that if you fill the jail with blocks and then hit R, you spawn outside the jail (since the inside is full of blocks). Is there something admins can do about this?
Yes, the admins can improve the architecture of their prison. Walls/floor/ceiling two blocks thick in every direction, the spawn point is set in the center on one cell, two high. You can't build if you're standing on the only space. Just in case, directly over the spawn point , build another 1x1x2 cell and cover it totally in adminium. That way, if some outside accomplice blocks up the entryway, griefers can't just re-enter the server and get out. Because the walls are so thick, speed hackers won't be able to glitch out of the corners.
That should about do it. Rogue OPs should be the only people that cause a problem with a prison like that.