How do I claim a 1099-S for a property I sold on land contract (and claimed that income in 2016), but they paid us off this year

[DianeW]

To record this on the tax return and eliminate an IRS letter asking about it later, enter the sale by using  the sales price and cost at the same amount to reflect a zero gain or loss.

This way the form is accounted for on the tax return while your previous return will reflect the reporting of the gain.  Maintain all documentation for your records should you need it later.