Remember that the forum mods are just regular players like you helping the community @Stefanakis.
As Nik said, only when you write for first time your Club Name, you are allowed to write numbers on it. If you Edit the Club Name after this point, the game don't allow this.
So there's no solution for now, apart from doing a account deletion, and start from 0, but that's a very radical option.
Cheers,