Sharepoint - How can I limit user to only enter one item in a list?

There are several way that you could do this. You could use an event receiver to enforce the constraint or you could use a field set to the value of the individual's user name and set it to be unique. You could do this with a workflow if you wanted a "code free" way of doing it. However, they could go back and change the value of this field.

In this case, I would likely use an event receiver as it would have fewer moving parts and could not be easily subverted but could still be disabled by an admin if required at some point.

As Dan suggested, it's better to use Calculated value as default:

Also, you can add this script to the NewForm.aspx:

$("[id^='Title_']").disabled = true

It will make sure the users cannot change the Title field:

Also, you can disable Quick Edit mode for the list to insure there is no easy workaround

I don't think using Workflows is an elegant solution

You could add a person/group column called 'AlreadyCreatedBy'. You could set that column to require unique values. Then, enable content approval on the list. Set up a workflow that copies 'Created By' to 'AlreadyCreatedBy' and if it succeeds then set the approval status of the item to approved.