Can I disable custom type-in amount field on my donation form?

Yes. You can disable the custom amount field on the campaign forms by going to the Amounts tab of the form editor. 

On the next page you can then mark the checkbox and click Update to disable this option:

Likewise, you can delete all the suggested amounts boxes so that the donor can only see the custom type-in field. 

After this option is selected, you will no longer see the "Custom Amount" textbox in your donation form.