Need to remove alias but is not work

Hi,

We suffered an attack for which I need to change the rights of a user who admin but impossible.
In addition I also need to delete an alias.
how could a user put such an alias on our server ?

cordially

Bad alias : 
https://riot.im/app/#/room/#sperme:matrix.org

Official Adresse is : https://riot.im/app/#/room/#linuxosfr:matrix.org