Reduce 2018 AGI by returning all SSA benifits?

I’m less than $2000 over the AGI limit for repaying all my healthcare subsidy. I can’t reduce my 2018 income by making an IRA contribution because we are both retired with no earned income. I’ve been getting SSA payments for less than six months, and I am still in the time limit to return all the SSA payments received and cancel my application. If I do that, does that reduce my 2018 income? Surely it doesn’t get processed as a credit on my 2019 income…I figure there isn’t time to complete the process by April 15 so I would file for an extension and pay tax based on what I expect to owe after returning the SSA income. Can I do this? Does anyone know any thing else I can do this year to reduce last year’s income?