drill a hole through the upright portion of the shelving unit - the piece the shelf portion of the french cleat is attached to: drill this hole so that it will penetrate the wall portion of the FC, preferably below the 45 cut. Then place a piece of dowel through the hole - make it a snug-but-not-too-tight fit. This will lock the shelf to the cleat: the shelf won't be able to move side to side, nor will it be subject to lifting forces, yet by simply pulling out the dowel you'll be able to move and rearrange the shelves. This requires, of course, that all your cleats be made to the same dimensions, at least as far as the dowel hole goes.
There is another way, but this is a lot more work, and reverses the normal positioning of the two parts of the FC, and requires two full cleats - one mounted upside down with respect to the other.
It also requires that the shelf be installed by sliding the shelf in from the side, not by lowering it from above.