You can indeed configure Lock to not show the sign up option (Lock Configuration Options), however, have in mind that this would purely an interface thing so technically sign ups would still be possible (for example, a technical user could call the sign up API directly).
In other words, depending on your exact scenario and related requirements there’s more to it than just disabling the option in the interface. For example, for a database connection you would need to use the disable sign ups toggle in the connection settings itself in order to disable public sign ups completely.