So I am hosting my own server and I banned someone but their TARDIS I can't seem to delete out of the overworld, is there anyway I can actually delete it?

To remove the physical exterior use:

/setblock x y z minecraft:air

if you have problems deleting the interior there should be  a way in /tardis-admin IF it is 1.12.

 It won't allow me to place it's saying 

os_zodiac issued server command: /setblock -84 64 228 minecraft:air

[13:57:52 INFO]: [os_zodiac: The block couldn't be placed]

[13:58:54 INFO]: The block couldn't be placed

Try installing world edit and doing //pos1 //pos2 around the tardis and then //set 0

^ do try installing world edit like 1WTC said. if the commands for some reason dont work try using a wooden axe on blocks close to the tardis

 Instead of using remove I used set owner thank you