Sorry - not completely familiar with 'over-jumping' term... So, does it mean the horse is taking off too far away or is jumping too high?
For both, I would definitely reccommend grid work. It gets the horse thinking and is a good brain work-out. Take-off poles, combinations and bounces, are all beneficial.
For taking off too far away... make sure that you are not sat too far forward (or adopting the jumping position too soon). I do that a lot, and am subconsciously preparing the horse too early. They take off far away to balance the 'over-forward' seat.
For jumping too high... I often find this happens with over-zealous or green horses. Sometimes, I'll incorporate some low fences in my flat work. For example, trotting/cantering serpentines, circles and figures of eight with jumps spaced throughout the route. When combined with some lateral work, it can really relax and collect an excitable horse.
Hope it helps!