TERMINATING THE PLAN (from a DOL publication)
QUOTE
Although SIMPLE IRA plans are established with the intention of being on-going, the time may come when a SIMPLE IRA plan no longer suits the purposes of your business. When that happens, consult with your financial institution to determine if another type of retirement plan might be a better alternative.
To terminate a SIMPLE IRA plan, notify the financial institution that you will not make a contribution for the next calendar year and that you want to terminate the contract or agreement. You must also notify your employees that the SIMPLE IRA plan will be discontinued.
You do not need to give any notice to the IRS that the SIMPLE IRA plan has been terminated.