Bukkit

This plugin gives four main features :

1. Disabling teleport and building on Nether Roof
2. Avoid generation on NetherRoof
3. Fix beacons in Nether : bedrock is replaced with barrier block, and replaced with bedrock again on beacon destroy.
4. Avoid fire formation from blazes and ghasts.

You can set many things in the config file :

checkpos-enabled: true # This will enable the check every x ticks specified bellow for all players in the Nether
checkpos-ticks : 40 # This is the delay for each check. 20 ticks = 1 sec, so default is 2 secs
checkpos-msg : "&cYou've been found on the Nether's roof. To prevent any chunk corruption, you're now at the world's spawn."
teleport-msg : "&cYour teleport to the Nether Roof has been canceled."
build-msg : "&cYou can't build on the Nether Roof."
no-fire-blaze : true # This will prevent blaze and ghast fire ignite
beacon-repair : true # This will replace bedrock above placed beacons with barrier block (1.8)
beacon-msg : "&cYou can't break this block as it has been replaced by the server." # This message appears if you try to replace the block replaced by the bedrock
beacon-sendmessage : true # Will send message to the player when placing the beacon
beacon-blocks-remove : "&cThere are <nbblocks> blocks that you must remove in order to make your beacon work. <avertlava>"
beacon-avertlava : true # The "<alertlava>" will be replaced if there is lava on the beacon's beam
beacon-lavamessage : "&cWarning ! There is lava out there !"

No permissions for now, if you realy need them comment ;)