Set up a trap with pistons in water, you can't teleport out if you're being shunted around, and they can't break the pistons either, the only choice they have is to either use /kill, or stop playing.
From what I understand breaking blocks would be against the rules even if you were stuck in a trap so water wouldn't be necessary. Messing around with the idea some I came up with these.
The timer is set to 0.7 seconds. A bit complicated and the loop on the left is arguably not necessary as you can't hear the timer ticking on SMP. The wires loop around the back to fire the piston on the left. If a block with a lever is placed on the orange block and the AND gate is set to accept input from all three sides the lever will act as a safety switch.
Edit: For some reason I simply cannot expand this beyond it's current size. It won't reset properly if I do. Fortunately you can remove a strip of blocks from the inside and it still works fine if a little slower.
Edit 2: This one has other problems so I wouldn't advise it now. Sometimes blocks get stuck. I'm working on a new version.
This one is much simpler but also much slower. The lava isn't necessary but more Fun.
I suggest these are put at the bottom of a drop long enough to stop quantum wearers from jumping out. Even with a closing roof we don't want someone else letting his chums out while falling in himself however unlikely.
It could easily be combined with
what I posted earlier.