You type in the command in chat /t unclaim while standing in the plot.
Chat with our AI personalities
There is no way to delete a plot, a.k.a. it isn't there. But, if you want to CLEAR it/not own in anymore, and the server is using the PlotMe plugin, and you have the correct permissions, you type in the chat /plotme reset
It is the same as survival, but the game is locked at the hardest difficulty, and you only have one life. When you die, you have to delete the world.
Just delete the folder with your Minecraft server stuff and you're done.
No
Unfortunately, there is no current way to delete a minecraft account. Unless, of course, you email the developers.