Adjust the lower(or upper) limiting screws to prevent the 10 speed derailleur from making that extra shift.

I'm planning to run 9 speed shimano with Campagnolo because
1. the cassette and chain last longer.
2. nobody makes a 135mm disc hub in Campagnolo
3. it's for a rain bike build
4. this bike is going to get a dynamo hub, so the shifting cables got to go.