My friends server keeps getting hacked

[LevelUpL]

My friends server keeps getting hacked 3 times in 1 month we dont know how these people are getting operator access to are console we also might have been doxed server owner Boggo_ My ign LevelUpL tell us if its a problem with are anti cheat grim or if this is happened to you please tell us how to fix it we also thinks its the same person there ign is kyuko_ 

[JoshG]

Are you using Vulcan Anti-Cheat? If so, remove it. It has a known backdoor giving users access to the server. Use the official one only https://www.spigotmc.org/resources/vulcan-anti-cheat-advanced-cheat-detection-1-7-1-20-1.83626/ found here. Or, use a different one.