As mentioned in this post, today the Purge starts! On the SMP server, PvP is turned on in the survival Overworld and Nether. You can destroy your own structures and if you are given permission, you can destroy other builds. While the Purge is active, the community rule of "don't be a jerk" is still enforced. Repeated killing after someone tells you to stop or destroying property without permission will result in a ban. At the spawn of SMP is a command block that will give you a stack of TNT to use. Next weekend, starting May 27 @ 1 PM Eastern US, the SMP spawn will be open to destruction.
On Modded, PvP will be allowed everywhere excpet for spawn. I can add a TNT command block if asked, though I know Modded has much more powerful tools. Let me know if you would like TNT there.