This is a classic case of poor watershed management. Here in California (the land of enviro-nazi's) this type of behavior does not fly. Construction sites that have turbid water discharging off site are subject to heavy fines by the Regional Water Quality Control Board. But I digress.
Hay bales are good but I find that fiber rolls are even better and easier to place and manipulate. Is the ditch dirt or concrete?
At the very least I would donate or ask the neighbors to pay for some fiber rolls to place around the sites that are the source of the erosion/sedimentation problem. You could also cover these spots with a tarp or plastic.
If your neighbors are not cooperative, do what you can in your own watershed. Slow the water down as best as possible with hay bales, fiber rolls, or any energy dissipation device. Basically we want to slow the water down so that the sediment has time to drop out before it reaches your pond.
A good tool for anyone who has erosion/sedimentation problems in their watershed is the Revised Universal Soil Loss Equation 2 (RUSLE2). Check out the link with the downloadable software.
http://fargo.nserl.purdue.edu/rusle2_dataweb/RUSLE2_Index.htm This software allows you to quantify how much soil loss or sedimentation will come off a given area. Then you can "plug in" best management practices (BMP's i.e. hay bales, fiber rolls, etc...) to see how much they will reduce the problem.
