the only way to slow down the spread is to apply quick pressure and make to make the offense take the 'hot-reads' and defense make tackles.... force the offense to dink-n-dunk all the way down field.
most spread teams are not patient and are prone to force one here or there....
so while they are taking what you give them, you continue to find the right MTM that will stop the deep ball.
one last thing, need to have your best defender deep to make the saving tackle.