So you are claiming that Killerash121 utilised an unknown exploit to steal stuff from your base, and your evidence for this is that you didn't leave your door open because you know you didn't and he said "yep" in chat?
The chances of him being able to use an exploit without being caught is minimal at best, especially if you actually do your base properly. Build a door + roof and there is no way for him to get in no matter what he does, no matter what "exploit" he uses.
Please either tell us what kind of exploit you believe he used, or gtfo. Just saying "he exploited lololol" does not constitute a ban.