This is possible, although I can’t say this is recommended. Here’s how to remove the last name field:
1) You go to Event Espresso > Registration Forms > Question Groups
2) Click the Personal Information group, you’ll be editing that group
3) Uncheck the box next to “Last Name” and click on Save
4) Then click on the Questions tab and go to edit the First Name question, you’ll change the question text to Name. Then click Save.
5) Test everything because you might find some unexpected quirks.