Like this?
If social security was shut down there's no reason not to continue paying senior citizens with what's left. Just incriment the age limit by one every year so only those currently getting checks from the government will get them from now on. Eventually those benefactors will kick off and the balance of the fund can be used to pay down the national debt or something like that. If it turns out that the fund is gonna dry up before the last benefactor dies, then don't end the tax completly, just reduce it annually as the population above the age limit shrinks; phase it out over time.