How do I disable drop-down login?

I notice that when I begin to log into any of my ww.wp.xz.cn sites, a drop-down box now appears. When I begin to type my username, the dropdown shows me all usernames and email addresses I’ve used to log in, beginning with that letter or number I’ve started to type.

I suppose this is for convenience, but I would like to disable this feature.

I feel it could be a way for someone to get my username (which is NOT admin).

How can I disable this dropdown box on my sites?

Thank you!