How to delete all messages from a person on server?

The problem is my server got attacked by various spammers, and big problems are:

1. I can't ban them through mobile application. They have white nicknames, they are not participants of the server, and somehow still can write in channels and tag everyone, when in settings there are not permission to write everyone_

When I try to tap on profile, I can ban only from my point of view, and there's no BAN button at all (I am an ADMIN, I can see other profiles normally for participants of server).

2. You can't delete ALL messages from a person at once. Even on PC version, you still should delete messages at once. Why? Why the Discord is so uncomfortable in terms of server management? I am so angry!!!