Comments section sign in

Hi, I am using Highend Childs Theme and have recently launched my website.

I have enabled the comments section on my posts (not pages) and want users to sign in with their name and e-mail. I have made sure these options are checked in Settings > discussion. However, I do not want the optional website box to be displayed as, when accessing the website on a mobile the three boxes (name, e-mail and website) all merge into each other and make it difficult to fill in any of them.

How do I get rid of the optional website box to leave just name and e-mail?

David