Minecraft – How to make members of a Team in Minecraft Invulnerable

Unfortunately on a server where there are no rules against griefing and everything is destroyable, there is no foolproof way to "trap" enemy players.

• Any redstone contraption can easily be avoided, as it requires stepping on a pressure plate which can be seen by anyone looking for them.

• Traps in pitch black can easily be avoided by small investments in torches by an opposing player.

• Any wall can be tunneled through.

• Aside from triggering redstone traps, the environment won't "update" in response to a player, thus any crafty player can avoid all traps with a little resources.

The best way to keep your stuff secure is to hide it. The best way to do this is to use a servers /home and /sethome commands. Simply go deep into the wilderness. Dig deep into the ground and ideally try to find a naturally occurring lava source. Replace the blocks above you and put your chest inside the lava source and /sethome. Even players using x-ray tools will only see the natural lava source (torches are a dead giveaway of a base) and won't be able to see the chest.

TL;DR: Bases can not be secure, and hiding is difficult from x-raying players. Put a chest in lava and walk away.

This is almost a version of @MrLemon's answer. Run the following command once:

/scoreboard objectives add Dimension dummy

Put the following commands on a fill/fast clock:

/scoreboard players set @a Dimension -2 {Dimension:-2}
/scoreboard players set @a Dimension -1 {Dimension:-1}
/scoreboard players set @a Dimension 0 {Dimension:0}    

Then do the tp as following:

/tp @a[score_tpHome_min=1,score_Dimension_min=0] 68 68 68

How this works is it gets the player dimension by setting the scoreboard objective Dimension to the dimension that they are in.