There was a ton of antimatter that obliterated all of the matter. I am in a DMND-FRME-TTAN room.

The next user is spammed with smileys.

I destroy the smileys by spamming anti-smileys.

I trapped the next user in a sealed container of superfluid helium at one degree kelvin. The user can only survive exposure to it for ten seconds, before they freeze to death.

Luckily I'm still wearing my space suit.

The next user is trapped in a box, locked in a cage, sealed in a tomb, cursed with a curse, barred by a door, held within a chamber, held within a pyramid, buried by sand, under the weight of a dune, located atop a mountain, in the centre isle of a volcano, at the end of the earth, surrounded by aliens. (of the evil kind)

There is no key.

I carefully use my teleport device to teleport almost out, all but my legs, which are stuck in the sand, i than pull out my singularity cannon and fire it at the aliens blowing them all up. Later that day i enjoy chinese food with Chuck Norris.

The next user is paralized in a pit of pit vipers, Wat do?

I am a blob of strange matter. As soon as I touch the ground, I convert it into more strange matter. I also converted the snakes into strange matter. Within a few seconds, I have converted the entire planet.

I touch the next user, converting them into more strange matter so quickly that they have no time to escape.

However, I am a blob of a more aggressive form of strange matter, and I fight it back.

The next user just pissed off a serial killer. And he is really, really, pissed.

Luckily, the serial killer is urophile.

The next user swims inside a closed cube of diamond, filled with water, conducting 20000 volts, 1000 ampere.

V = IR so the resistance in the cube would equate to only 2 Ohms. This is ridiculous, and it turns out there was a miscalculation, and the current passing though was only  0.002 amps, as the resistance in the pure distilled water is about 10 mega Ohms. (Neglecting my resistance)

The next user is in the same scenario but the current is correct, and is 1000 amps. (the maximum safe amount of current passing though the body is 8 milli Amps/0.008 Amps.)

However, the electrical resistance of my body is higher than that of the water, so the electricity simply goes around me. It als splits the water into hydrogen and oxygen, thus wasting most of its energy even before it reaches me.

That's not an attack.....

___________________________________________________________________________________________

After my first attempt failed, I re-assess my prison features, and add some more...
The next user is:

1. Trapped in a box 
2. Covered with concrete
3. perminently frozen dead
4. locked in a cage
5. sealed in a tomb
6. cursed with a curse
7. barred by a door
8. held within a chamber
9. held within a pyramid
10. buried by sand
11. under the weight of a dune
12. in the middle of an unknown desert
13. surrounded by mountains
14. on a volcanic fault line
15. at the end of the earth
16. Beneath a giant dome
17. with an anti teleport barrier
18. sealed airtight
19. surrounded by aliens
20. dumped in a distant universe (and the earth with you)
21. in the centre of a giant star
22. that’s about to supernova
23. surrounded by more aliens (of the evil kind)
24. trapped in one moment of time
25. Out of the reach of doctor who
26. on the event horizon of a black hole
27. in another dimension