On Delta Chat, your identity is tied to your public key cryptographical key pair, not to any single relay you use to send and receive messages per se. I.e., the hash fingerprint is the identifier, not the email address.
This means even if you lock out a malicious device from a single mailbox, they can just add another relay and keep sending messages in your name (including ones that induce others to route their private and group messages towards the malicious actor, not the updated mailbox) and if they ever get into a MITM position may also be able to decrypt all incoming messages as long as forward secrecy is not implemented. You will have better luck with a workaround such as: