Unfortunately, if you want to get down to clear soil you're going to have to lift the turf off the top. If your garden is reasonably small its possible to do this with a spade and wheel barrow....and a little muscle power. You'd be surprised how much you'll get done in a weekend. If the garden is too big for this, maybe rent mini-digger of some variety?
Either way, you will need to lift probably the top 6 inches of soil/turf. This will bring you down to clear soil. If you decide to do a little landscaping and put split levels in your garden, the turf you lifted is ideal to fill up the raised areas. Alternatively, you can pile it up at the back of the garden for a couple of months and use it in flower beds/borders. (We did this a couple of years ago. Didn't use the turf for a year, but built a raised Veg. Garden the following year....all roots and grass had rotted by then)
Rotavating the soil will make life somewhat difficult when you go to seed, as you'll have to rake out all the dead grass which you'll have just turned over. Its still an option if you don't want to dig it out.