To choose which services a user can access, go to the Services page in that user’s account, under the Users tab. There you can hand-pick, on a per-user basis, exactly which services and features the user is allowed to use, such as SFTP, FTP/FTPS, and HTTPS. When you’ve made your selection, click Apply or OK to save.
Because services are set per user, different accounts can have different access on the same server, so you might give one user SFTP only while another also has the HTTPS web interface. Note that a service must also be enabled on the server itself before any user can use it; the per-user Services page controls who gets access to the services you’ve turned on.