account impersonation

[Vega Slayer]

So, the username AvA used my account name twice yesterday, first time i caught him using it because ive noticed my name playing on the server on the website and it wasnt me, and when i logged in it said my nick is "Unnamedplayer" and not my regular username, i told him to stop and he changed his name back. But it happened again. Once more experiencing loging in as unnamedplayer and his "quotes" that i didnt say
I was wondering, why is it allowed for someone to steal account names, is there a way to prevent this?
some of screenshots:
https://ibb.co/RzQgKBn
https://ibb.co/56m83jj

[Vega Slayer]

wanted to give them another chance though, hopefully they wont do it again, in case they are willing to stop

[adminless]

I can register the 'Vega Slayer' nick if you want though for that you either need a forum account or to contact me directly to receive the registration instructions obviously.