How do I stop a master from being a master in mariadb
There's RESET MASTER, but that just removes the binary logs and resets the binlog file number to 1. It doesn't prevent replicas from connecting. But they will be very confused if they connect and the binlog file number has been reset.
You could revoke REPLICATION SLAVE privilege from the username used by any of the replicas that are currently connecting.
Or you could disable binary logs, which is accomplished by removing the log_bin option from your server options file and restarting. AFAIK, you cannot disable the binary log dynamically.