In 2014, I operated as a sole proprietor. In December 2014, I ceased operations and took a full-time, salaried, W2 job.
I'm under contract on a home now in 2015, and the USDA (it's a USDA Rural Development Loan) is stating that they must use my 2014 Schedule C income to predict my 2015 total income, putting me way over the limit for USDA's RDL income limit.

How can I prove (apart from a written statement I've already submitted) that I did not operate a Schedule C/sole proprietorship in 2015, and that I made no money from anything but my current W2 job?

How can I prove (apart from a written statement I've already submitted) that I did not operate a Schedule C/sole proprietorship in 2015, and that I made no money from anything but my current W2 job?=====>>I guess you may , along with a written stmt again with the all zero income/expense on yoru 2015 Sch C of 1040/ all zero income/expense on Sch SE of 1040. Or you just submit them a copy of your 2015 fed/state returns along with the stmt again as there is NO penalty in submitting a stmt .In my opinion, before doing this, you need to contact the USDA financial dept for asdvise.