Have you had something that you really want to protect but your buddy keeps stealing it.

with this command not even creative mode players can get in

You need a command block for this set on repeat, always active, and unconditional

/fill ~ ~ ~ ~ ~ ~ obsidian

~= Coordinate number

T he first 3 ~ is your starting coordinate and the last 3 ~ is the last coordinate

all space inbetween will be regenerating obsidian